avutil/log: print level prefix also when no AVClass context is available

Adds the level prefix to all log messages, except those with level <=
AV_LOG_QUIET as they seem to be used for flushing the log buffer.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Tobias Rapp <t.rapp@noa-archive.com>

libavutil/log.c

@@ -266,10 +266,10 @@ static void format_line(void *avcl, int level, const char *fmt, va_list vl,
         av_bprintf(part+1, "[%s @ %p] ",
                  avc->item_name(avcl), avcl);
         if(type) type[1] = get_category(avcl);
+    }
 
-        if (flags & AV_LOG_PRINT_LEVEL)
+    if (*print_prefix && (level > AV_LOG_QUIET) && (flags & AV_LOG_PRINT_LEVEL))
         av_bprintf(part+2, "[%s] ", get_level_str(level));
-    }
 
     av_vbprintf(part+3, fmt, vl);