app_mp3: Throw warning on nonexistent stream

Currently, the MP3Player application doesn't
emit a warning if attempting to play a stream
which no longer exists. This can be a common
scenario as many mp3 streams are valid at some
point but can disappear at any time.

Now a warning is thrown if attempting to play
a nonexistent MP3 stream, instead of silently
exiting.

ASTERISK-29829 #close

Change-Id: I53a0bf1ed1740166655eb66fe7675f6f808bf535

apps/app_mp3.c

@@ -179,6 +179,7 @@ static int mp3_exec(struct ast_channel *chan, const char *data)
 	int pid = -1;
 	RAII_VAR(struct ast_format *, owriteformat, NULL, ao2_cleanup);
 	int timeout = 2;
+	int startedmp3 = 0;
 	struct timeval next;
 	struct ast_frame *f;
 	struct myframe {
@@ -242,12 +243,16 @@ static int mp3_exec(struct ast_channel *chan, const char *data)
 				if (res > 0) {
 					myf.f.datalen = res;
 					myf.f.samples = res / 2;
+					startedmp3 = 1;
 					if (ast_write(chan, &myf.f) < 0) {
 						res = -1;
 						break;
 					}
 				} else {
 					ast_debug(1, "No more mp3\n");
+					if (!startedmp3) { /* we couldn't do anything, which means this stream doesn't work */
+						ast_log(LOG_WARNING, "MP3 stream '%s' is broken or nonexistent\n", data);
+					}
 					res = 0;
 					break;
 				}