Fix wrong zmalloc_size() assumption. (#7963)

When using a system with no malloc_usable_size(), zmalloc_size() assumed
that the heap allocator always returns blocks that are long-padded.

This may not always be the case, and will result with zmalloc_size()
returning a size that is bigger than allocated. At least in one case
this leads to out of bound write, process crash and a potential security
vulnerability.

Effectively this does not affect the vast majority of users, who use
jemalloc or glibc.

This problem along with a (different) fix was reported by Drew DeVault.

src/zmalloc.c

@@ -236,9 +236,6 @@ void *zrealloc_usable(void *ptr, size_t size, size_t *usable) {
 size_t zmalloc_size(void *ptr) {
     void *realptr = (char*)ptr-PREFIX_SIZE;
     size_t size = *((size_t*)realptr);
-    /* Assume at least that all the allocations are padded at sizeof(long) by
-     * the underlying allocator. */
-    if (size&(sizeof(long)-1)) size += sizeof(long)-(size&(sizeof(long)-1));
     return size+PREFIX_SIZE;
 }
 size_t zmalloc_usable_size(void *ptr) {