Make av_strerror() print an error message mentioning the error code

number if strerror_r() did not succeed for whatever reason.

This avoids the need for the application to fill the string in case
strerror_r() fails, for example because the error code is not known.

Originally committed as revision 23015 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/error.c

@@ -36,9 +36,9 @@ int av_strerror(int errnum, char *errbuf, size_t errbuf_size)
     } else {
 #if HAVE_STRERROR_R
         ret = strerror_r(AVUNERROR(errnum), errbuf, errbuf_size);
-#else
-        snprintf(errbuf, errbuf_size, "Error number %d occurred", errnum);
 #endif
+        if (!HAVE_STRERROR_R || ret < 0)
+            snprintf(errbuf, errbuf_size, "Error number %d occurred", errnum);
     }
 
     return ret;