Fix memory corruption when running with FSA_SIZE_DEBUG set

dbg->memory is not a real pointer, it's just the placeholder for the actual
memory area (that's preceeded by the allocated size). Also we don't have to
offset (char *) by the size field as the allocator will already return the
pointer to ->memory, so *p is already the start of the actual data.

Thanks to dogbert17++ for pointing this out!

src/core/fixedsizealloc.c

@@ -231,7 +231,7 @@ void * MVM_fixed_size_realloc(MVMThreadContext *tc, MVMFixedSizeAlloc *al, void
 void * MVM_fixed_size_realloc_at_safepoint(MVMThreadContext *tc, MVMFixedSizeAlloc *al, void * p, size_t old_bytes, size_t new_bytes) {
 #if FSA_SIZE_DEBUG
     MVMFixedSizeAllocDebug *dbg = MVM_fixed_size_alloc(tc, al, new_bytes);
-    memcpy(dbg->memory, (char *)p + sizeof(MVMuint64), new_bytes > old_bytes ? old_bytes : new_bytes);
+    memcpy(&(dbg->memory), (char *)p, new_bytes > old_bytes ? old_bytes : new_bytes);
     MVM_fixed_size_free_at_safepoint(tc, al, old_bytes, p);
     dbg->alloc_size = new_bytes;
     return &(dbg->memory);