avcodec/scpr: Fix use of uninitialized variable

Fixes: Undefined shift
Fixes: 12911/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SCPR_fuzzer-5677102915911680

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 53248acfb3)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/scpr.c

@@ -504,7 +504,7 @@ static int decompress_p(AVCodecContext *avctx,
 {
     SCPRContext *s = avctx->priv_data;
     GetByteContext *gb = &s->gb;
-    int ret, temp, min, max, x, y, cx = 0, cx1 = 0;
+    int ret, temp = 0, min, max, x, y, cx = 0, cx1 = 0;
     int backstep = linesize - avctx->width;
     const int cxshift = s->cxshift;