wav: propagate ff_get_wav_header() error code in w64_read_header()

Alos fix warning:
wav.c: In function ‘w64_read_header’:
wav.c:546: warning: ‘ret’ may be used uninitialized in this function

libavformat/wav.c

@@ -194,7 +194,7 @@ static int wav_parse_fmt_tag(AVFormatContext *s, int64_t size, AVStream **st)
     if (!*st)
         return AVERROR(ENOMEM);
 
-    ff_get_wav_header(pb, (*st)->codec, size);
+    ret = ff_get_wav_header(pb, (*st)->codec, size);
     if (ret < 0)
         return ret;
     (*st)->need_parsing = AVSTREAM_PARSE_FULL;