avutil/log: dont count&print repeats for empty strings

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/log.c

@@ -222,7 +222,7 @@ void av_log_default_callback(void* ptr, int level, const char* fmt, va_list vl)
         is_atty = isatty(2) ? 1 : -1;
 #endif
 
-    if (print_prefix && (flags & AV_LOG_SKIP_REPEATED) && !strcmp(line, prev)){
+    if (print_prefix && (flags & AV_LOG_SKIP_REPEATED) && !strcmp(line, prev) && *line){
         count++;
         if (is_atty == 1)
             fprintf(stderr, "    Last message repeated %d times\r", count);