avutil/log: leave prefix printing state on empty input.

Fixes printing the prefix after an empty string was printed

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

libavutil/log.c

@@ -190,7 +190,8 @@ static void format_line(void *ptr, int level, const char *fmt, va_list vl,
 
     vsnprintf(part[2], part_size, fmt, vl);
 
-    *print_prefix = strlen(part[2]) && part[2][strlen(part[2]) - 1] == '\n';
+    if(*part[0] || *part[1] || *part[2])
+        *print_prefix = strlen(part[2]) && part[2][strlen(part[2]) - 1] == '\n';
 }
 
 void av_log_format_line(void *ptr, int level, const char *fmt, va_list vl,