Make av_strerror() return -1 even in the case when av_strerror_r() is

not defined.

This allows applications to check if av_strerror() cannot provide a
meaningful representation for the provided error code, without having
to actually check the filled string.

Originally committed as revision 23031 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/error.c

@@ -36,8 +36,10 @@ 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
+        ret = -1;
 #endif
-        if (!HAVE_STRERROR_R || ret < 0)
+        if (ret < 0)
             snprintf(errbuf, errbuf_size, "Error number %d occurred", errnum);
     }