Fix the real allocation size in OpenBSD rallocx shim

In the rallocx() shim for OpenBSD (that's the only platform that doesn't
have malloc_size() or malloc_usable_size() equivalent), the newly
allocated size was missing the extra size_t member for storing the
allocation size leading to size_t sized overflow at the end of the
reallocated memory chunk.

lib/isc/jemalloc_shim.h

@@ -106,7 +106,7 @@ static inline void *
 rallocx(void *ptr, size_t size, int flags) {
 	UNUSED(flags);
 
-	size_t *__ptr = realloc(&((size_t *)ptr)[-1], size);
+	size_t *__ptr = realloc(&((size_t *)ptr)[-1], size + sizeof(size_t));
 	REQUIRE(__ptr != NULL);
 	__ptr[0] = size;