lavu/error: fix sign error in av_sterror() error code

The value returned by strerror_r may be positive depending on the system,
so it is necessary to use AVERROR() in order to get a negative
number. This way the successive negative check will not fail, and the
function will print a meaningful error message on the buffer.

libavutil/error.c

@@ -61,7 +61,7 @@ int av_strerror(int errnum, char *errbuf, size_t errbuf_size)
         av_strlcpy(errbuf, entry->str, errbuf_size);
     } else {
 #if HAVE_STRERROR_R
-        ret = strerror_r(AVUNERROR(errnum), errbuf, errbuf_size);
+        ret = AVERROR(strerror_r(AVUNERROR(errnum), errbuf, errbuf_size));
 #else
         ret = -1;
 #endif