avcodec/smcenc: make sure ny/nx are >= 0

(cherry picked from commit 5ad38785e7ad4067a288e9d5e8ce2c4ed2bf584a)

Fixes: out of array read
Fixes: 68939/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SMC_fuzzer-587804104884224

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

libavcodec/smcenc.c

@@ -184,8 +184,8 @@ static void smc_encode_stream(SMCContext *s, const AVFrame *frame,
             const ptrdiff_t offset = xpixel_ptr - src_pixels;
             const int sy = offset / stride;
             const int sx = offset % stride;
-            const int ny = sx < 4 ? sy - 4 : sy;
-            const int nx = sx < 4 ? width - 4 + (width & 3) : sx - 4;
+            const int ny = sx < 4 ? FFMAX(sy - 4, 0) : sy;
+            const int nx = sx < 4 ? FFMAX(width - 4 + (width & 3), 0) : sx - 4;
             const uint8_t *old_pixel_ptr = src_pixels + nx + ny * stride;
             int compare = 0;