avformat/apngdec: Use 64bit for ret to avoid overflow

Missed this hunk in first commit
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 80a851aa)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/apngdec.c

@@ -150,7 +150,8 @@ static int apng_read_header(AVFormatContext *s)
     AVIOContext *pb = s->pb;
     uint32_t len, tag;
     AVStream *st;
-    int ret = AVERROR_INVALIDDATA, acTL_found = 0;
+    int acTL_found = 0;
+    int64_t ret = AVERROR_INVALIDDATA;
 
     /* verify PNGSIG */
     if (avio_rb64(pb) != PNGSIG)