avcodec/pgxdec: Make better use of size check

Each of the three calls to pgx_get_number() consumes at least two bytes.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/pgxdec.c

@@ -56,9 +56,8 @@ static int pgx_decode_header(AVCodecContext *avctx, GetByteContext *g,
 {
     int byte;
 
-    if (bytestream2_get_bytes_left(g) < 6) {
+    if (bytestream2_get_bytes_left(g) < 12)
         return AVERROR_INVALIDDATA;
-    }
 
     bytestream2_skip(g, 6);