avcodec/avcodec: don't print coded dimensions if not set

The avctx passed to avcodec_string() may have unset coded dimensions, as is the
case when called by av_dump_format() where the streams had all the needed
information at the container level, and as such no frames were decoded
internally.

Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/avcodec.c

@@ -625,6 +625,7 @@ void avcodec_string(char *buf, int buf_size, AVCodecContext *enc, int encode)
                        enc->width, enc->height);
 
             if (av_log_get_level() >= AV_LOG_VERBOSE &&
+                enc->coded_width && enc->coded_height &&
                 (enc->width != enc->coded_width ||
                  enc->height != enc->coded_height))
                 av_bprintf(&bprint, " (%dx%d)",