swscale/utils: clear formatConvBuffer on allocation

Fixes use of uninitialized memory
Fixes: asan_heap-oob_35ca682_1474_cov_3230122439_aletrek_tga_16bit.mov

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 007498fc)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/utils.c

@@ -1314,7 +1314,7 @@ av_cold int sws_init_context(SwsContext *c, SwsFilter *srcFilter,
     c->chrDstW = FF_CEIL_RSHIFT(dstW, c->chrDstHSubSample);
     c->chrDstH = FF_CEIL_RSHIFT(dstH, c->chrDstVSubSample);
 
-    FF_ALLOC_OR_GOTO(c, c->formatConvBuffer, FFALIGN(srcW*2+78, 16) * 2, fail);
+    FF_ALLOCZ_OR_GOTO(c, c->formatConvBuffer, FFALIGN(srcW*2+78, 16) * 2, fail);
 
     /* unscaled special cases */
     if (unscaled && !usesHFilter && !usesVFilter &&