fix realloc from small sized allocations with above PAGE_SIZE alignment

Large allocations don't always have a size larger than the maximum slab
size class because alignment larger than PAGE_SIZE is handled via large
allocations. The general case in realloc was assuming small sizes imply
slab allocations which isn't guaranteed.

In practice, large alignments are extremely rare. Since realloc doesn't
preserve alignment, combining both together doesn't happen in practice
which is why this issue wasn't noticed.

Reported-by: Stefan Rus <stefan@photonspark.com>

Author: thestinger

h_malloc.c

@@ -1530,7 +1530,8 @@ EXPORT void *h_realloc(void *old, size_t size) {
     old = untag_pointer(old);
 
     size_t old_size;
-    if (old < get_slab_region_end() && old >= ro.slab_region_start) {
+    bool old_in_slab_region = old < get_slab_region_end() && old >= ro.slab_region_start;
+    if (old_in_slab_region) {
         old_size = slab_usable_size(old);
         if (size <= max_slab_size_class && get_size_info(size).size == old_size) {
             return old_orig;
@@ -1647,7 +1648,7 @@ EXPORT void *h_realloc(void *old, size_t size) {
         copy_size -= canary_size;
     }
     memcpy(new, old_orig, copy_size);
-    if (old_size <= max_slab_size_class) {
+    if (old_in_slab_region) {
         deallocate_small(old, NULL);
     } else {
         deallocate_large(old, NULL);