swscale/slice: clear allocated memory in alloc_lines()

Fixes: use of uninitialized memory in hScale16To15_c()
Fixes: 373924007/clusterfuzz-testcase-minimized-ffmpeg_SWS_fuzzer-5841199968092160

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 aeec39f3c1)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/slice.c

@@ -60,7 +60,7 @@ static int alloc_lines(SwsSlice *s, int size, int width)
         for (j = 0; j < n; ++j) {
             // chroma plane line U and V are expected to be contiguous in memory
             // by mmx vertical scaler code
-            s->plane[i].line[j] = av_malloc(size * 2 + 32);
+            s->plane[i].line[j] = av_mallocz(size * 2 + 32);
             if (!s->plane[i].line[j]) {
                 free_lines(s);
                 return AVERROR(ENOMEM);