amrwb: error out early if mode is invalid.

Prevents using the invalid mode as an index in a static array, which
would generate invalid reads.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/amrwbdec.c

@@ -1095,23 +1095,27 @@ static int amrwb_decode_frame(AVCodecContext *avctx, void *data,
     buf_out = (float *)ctx->avframe.data[0];
 
     header_size      = decode_mime_header(ctx, buf);
+    if (ctx->fr_cur_mode > MODE_SID) {
+        av_log(avctx, AV_LOG_ERROR,
+               "Invalid mode %d\n", ctx->fr_cur_mode);
+        return AVERROR_INVALIDDATA;
+    }
     expected_fr_size = ((cf_sizes_wb[ctx->fr_cur_mode] + 7) >> 3) + 1;
 
     if (buf_size < expected_fr_size) {
         av_log(avctx, AV_LOG_ERROR,
             "Frame too small (%d bytes). Truncated file?\n", buf_size);
         *got_frame_ptr = 0;
-        return buf_size;
+        return AVERROR_INVALIDDATA;
     }
 
     if (!ctx->fr_quality || ctx->fr_cur_mode > MODE_SID)
         av_log(avctx, AV_LOG_ERROR, "Encountered a bad or corrupted frame\n");
 
-    if (ctx->fr_cur_mode == MODE_SID) /* Comfort noise frame */
+    if (ctx->fr_cur_mode == MODE_SID) { /* Comfort noise frame */
         av_log_missing_feature(avctx, "SID mode", 1);
-
-    if (ctx->fr_cur_mode >= MODE_SID)
         return -1;
+    }
 
     ff_amr_bit_reorder((uint16_t *) &ctx->frame, sizeof(AMRWBFrame),
         buf + header_size, amr_bit_orderings_by_mode[ctx->fr_cur_mode]);