mpeg4audio: check the init_get_bits() return value

Fixes possible invalid reads.

CC:libav-stable@libav.org

libavcodec/mpeg4audio.c

@@ -80,9 +80,11 @@ int avpriv_mpeg4audio_get_config(MPEG4AudioConfig *c, const uint8_t *buf,
                                  int bit_size, int sync_extension)
 {
     GetBitContext gb;
-    int specific_config_bitindex;
+    int specific_config_bitindex, ret;
 
-    init_get_bits(&gb, buf, bit_size);
+    ret = init_get_bits(&gb, buf, bit_size);
+    if (ret < 0)
+        return ret;
     c->object_type = get_object_type(&gb);
     c->sample_rate = get_sample_rate(&gb, &c->sampling_index);
     c->chan_config = get_bits(&gb, 4);