avcodec/ac3_parser: Check init_get_bits8() for failure

Fixes: null pointer dereference
Fixes: ffmpeg_crash_6.avi

Found-by: Thuan Pham, Marcel Böhme, Andrew Santosa and Alexandru Razvan Caciulescu with AFLSmart
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/ac3_parser.c

@@ -162,7 +162,9 @@ int avpriv_ac3_parse_header(AC3HeaderInfo **phdr, const uint8_t *buf,
         return AVERROR(ENOMEM);
     hdr = *phdr;
 
-    init_get_bits8(&gb, buf, size);
+    err = init_get_bits8(&gb, buf, size);
+    if (err < 0)
+        return AVERROR_INVALIDDATA;
     err = ff_ac3_parse_header(&gb, hdr);
     if (err < 0)
         return AVERROR_INVALIDDATA;