avcodec/tmv: Move space check before buffer allocation

Fixes: Timeout
Fixes: 471664630/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_TMV_fuzzer-5291752530706432

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/tmv.c

@@ -47,9 +47,6 @@ static int tmv_decode_frame(AVCodecContext *avctx, AVFrame *frame,
     unsigned x, y, fg, bg, c;
     int ret;
 
-    if ((ret = ff_get_buffer(avctx, frame, 0)) < 0)
-        return ret;
-
     if (avpkt->size < 2*char_rows*char_cols) {
         av_log(avctx, AV_LOG_ERROR,
                "Input buffer too small, truncated sample?\n");
@@ -57,6 +54,9 @@ static int tmv_decode_frame(AVCodecContext *avctx, AVFrame *frame,
         return AVERROR_INVALIDDATA;
     }
 
+    if ((ret = ff_get_buffer(avctx, frame, 0)) < 0)
+        return ret;
+
     dst              = frame->data[0];
 
     memcpy(frame->data[1], ff_cga_palette, 16 * 4);