swscale/utils: Clear pix buffers

Fixes use of uninitialized memory
Fixes: a96874b9466b6edc660a519c7ad47977_signal_sigsegv_7ffff713351a_744_nc_sample.avi with memlimit 2147483648

Found-by: Samuel Groß, Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/utils.c

@@ -1496,9 +1496,9 @@ av_cold int sws_init_context(SwsContext *c, SwsFilter *srcFilter,
 
     /* Allocate pixbufs (we use dynamic allocation because otherwise we would
      * need to allocate several megabytes to handle all possible cases) */
-    FF_ALLOC_OR_GOTO(c, c->lumPixBuf,  c->vLumBufSize * 3 * sizeof(int16_t *), fail);
-    FF_ALLOC_OR_GOTO(c, c->chrUPixBuf, c->vChrBufSize * 3 * sizeof(int16_t *), fail);
-    FF_ALLOC_OR_GOTO(c, c->chrVPixBuf, c->vChrBufSize * 3 * sizeof(int16_t *), fail);
+    FF_ALLOCZ_OR_GOTO(c, c->lumPixBuf,  c->vLumBufSize * 3 * sizeof(int16_t *), fail);
+    FF_ALLOCZ_OR_GOTO(c, c->chrUPixBuf, c->vChrBufSize * 3 * sizeof(int16_t *), fail);
+    FF_ALLOCZ_OR_GOTO(c, c->chrVPixBuf, c->vChrBufSize * 3 * sizeof(int16_t *), fail);
     if (CONFIG_SWSCALE_ALPHA && isALPHA(c->srcFormat) && isALPHA(c->dstFormat))
         FF_ALLOCZ_OR_GOTO(c, c->alpPixBuf, c->vLumBufSize * 3 * sizeof(int16_t *), fail);
     /* Note we need at least one pixel more at the end because of the MMX code