pickcdef,av1_cdef_search: use calloc for cdef_search_ctx

Avoids free's of uninitialized pointer should any subsequent allocations
fail in `cdef_alloc_data()`. Needed after:
4d28dff725 Refactor memory alloc/dealloc of CDEF search context buffers

Bug: 501471698
Fixed: 501471698
Change-Id: I4a9b52442daa6523e3302cae83fc02abf44358c9

av1/encoder/pickcdef.c

diff --git a/av1/encoder/pickcdef.c b/av1/encoder/pickcdef.c
index d498c06..a315f6a 100644
--- a/av1/encoder/pickcdef.c
+++ b/av1/encoder/pickcdef.c
@@ -874,7 +874,7 @@
 
   if (!cpi->cdef_search_ctx)
     CHECK_MEM_ERROR(cm, cpi->cdef_search_ctx,
-                    aom_malloc(sizeof(*cpi->cdef_search_ctx)));
+                    aom_calloc(1, sizeof(*cpi->cdef_search_ctx)));
   CdefSearchCtx *cdef_search_ctx = cpi->cdef_search_ctx;
 
   // Initialize parameters related to CDEF search context.