make map_name() interpret empty strings as "no separator"

empty strings were previously meaningless, and starting with 72c2d695a,
failure to handle them lead to bogus results when the IMAP hierarchy
separator is legitimately empty (when the server genuinely supports none
and none is manually configured). non-null can be asserted more cleanly
than null-or-non-empty, so change the api like that.
incidentally, this also removes the need to work around gcc's bogus
warning in -Os mode.

problem found by "Casper Ti. Vector" <caspervector@gmail.com>

src/config.c

@@ -355,6 +355,8 @@ load_config( const char *where, int pseudo )
 				if (store) {
 					if (!store->max_size)
 						store->max_size = INT_MAX;
+					if (!store->flat_delim)
+						store->flat_delim = "";
 					*storeapp = store;
 					storeapp = &store->next;
 					*storeapp = 0;

src/main.c

@@ -983,7 +983,7 @@ store_connected( int sts, void *aux )
 							flags |= LIST_INBOX;
 						} else if (c == '/') {
 							/* Flattened sub-folders of INBOX actually end up in Path. */
-							if (mvars->ctx[t]->conf->flat_delim)
+							if (mvars->ctx[t]->conf->flat_delim[0])
 								flags |= LIST_PATH;
 							else
 								flags |= LIST_INBOX;
@@ -1027,7 +1027,7 @@ store_listed( int sts, string_list_t *boxes, void *aux )
 		return;
 	case DRV_OK:
 		for (box = boxes; box; box = box->next) {
-			if (mvars->ctx[t]->conf->flat_delim) {
+			if (mvars->ctx[t]->conf->flat_delim[0]) {
 				string_list_t *nbox;
 				if (map_name( box->string, (char **)&nbox, offsetof(string_list_t, string), mvars->ctx[t]->conf->flat_delim, "/" ) < 0) {
 					error( "Error: flattened mailbox name '%s' contains canonical hierarchy delimiter\n", box->string );

src/sync.c

@@ -1014,7 +1014,7 @@ sync_boxes( store_t *ctx[], const char *names[], int present[], channel_conf_t *
 		svars->orig_name[t] =
 			(!names[t] || (ctx[t]->conf->map_inbox && !strcmp( ctx[t]->conf->map_inbox, names[t] ))) ?
 				"INBOX" : names[t];
-		if (!ctx[t]->conf->flat_delim) {
+		if (!ctx[t]->conf->flat_delim[0]) {
 			svars->box_name[t] = nfstrdup( svars->orig_name[t] );
 		} else if (map_name( svars->orig_name[t], &svars->box_name[t], 0, "/", ctx[t]->conf->flat_delim ) < 0) {
 			error( "Error: canonical mailbox name '%s' contains flattened hierarchy delimiter\n", svars->orig_name[t] );

src/util.c

@@ -479,19 +479,20 @@ map_name( const char *arg, char **result, int reserve, const char *in, const cha
 	char *p;
 	int i, l, ll, num, inl, outl;
 
+	assert( arg );
 	l = strlen( arg );
-	if (!in) {
+	assert( in );
+	inl = strlen( in );
+	if (!inl) {
 	  copy:
 		*result = nfmalloc( reserve + l + 1 );
 		memcpy( *result + reserve, arg, l + 1 );
 		return 0;
 	}
-	inl = strlen( in );
-	if (out) {
+	assert( out );
 	outl = strlen( out );
 	if (inl == outl && !memcmp( in, out, inl ))
 		goto copy;
-	}
 	for (num = 0, i = 0; i < l; ) {
 		for (ll = 0; ll < inl; ll++)
 			if (arg[i + ll] != in[ll])
@@ -500,7 +501,7 @@ map_name( const char *arg, char **result, int reserve, const char *in, const cha
 		i += inl;
 		continue;
 	  fout:
-		if (out) {
+		if (outl) {
 			for (ll = 0; ll < outl; ll++)
 				if (arg[i + ll] != out[ll])
 					goto fnexti;
@@ -511,7 +512,7 @@ map_name( const char *arg, char **result, int reserve, const char *in, const cha
 	}
 	if (!num)
 		goto copy;
-	if (!out)
+	if (!outl)
 		return -2;
 	*result = nfmalloc( reserve + l + num * (outl - inl) + 1 );
 	p = *result + reserve;
@@ -519,15 +520,7 @@ map_name( const char *arg, char **result, int reserve, const char *in, const cha
 		for (ll = 0; ll < inl; ll++)
 			if (arg[i + ll] != in[ll])
 				goto rnexti;
-#if defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 3)) && !defined(__clang__)
-# pragma GCC diagnostic push
-/* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=42145 */
-# pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
-#endif
 		memcpy( p, out, outl );
-#if defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 3)) && !defined(__clang__)
-# pragma GCC diagnostic pop
-#endif
 		p += outl;
 		i += inl;
 		continue;