force a GC in allocate3 if the heap limit has been exceeded

Otherwise, we'll throw an OOME even though there may be enough
unreachable objects eligible for collection to get back below the
limit.

src/machine.cpp

@@ -3519,7 +3519,7 @@ allocate3(Thread* t, Allocator* allocator, Machine::AllocationType type,
       break;
     }
 
-    if (t->heap == 0) {
+    if (t->heap == 0 or t->m->heap->limitExceeded()) {
       //     fprintf(stderr, "gc");
       //     vmPrintTrace(t);
       collect(t, Heap::MinorCollection);