malloc --> calloc in H5B cache entry (#5053)

oss-fuzz occasionally complains about intermittent undefined behavior
when when Boolean variables in H5C_cache_entry_t (like is_protected)
are set to values other than 0 or 1 when parsing fuzzed files. Using
calloc() to iniitialize the H5B_t struct (like other metadata already
does) should fix this.

src/H5B.c

@@ -215,7 +215,7 @@ H5B_create(H5F_t *f, const H5B_class_t *type, void *udata, haddr_t *addr_p /*out
     /*
      * Allocate file and memory data structures.
      */
-    if (NULL == (bt = H5FL_MALLOC(H5B_t)))
+    if (NULL == (bt = H5FL_CALLOC(H5B_t)))
         HGOTO_ERROR(H5E_BTREE, H5E_CANTALLOC, FAIL, "memory allocation failed for B-tree root node");
     memset(&bt->cache_info, 0, sizeof(H5AC_info_t));
     bt->level     = 0;