update Context::executableSize after allocating objectPool

This ensures that, if an exception is thrown later but before the
method has been fully compiled, we will know exactly how much memory
to free.  Previously, we would abort when trying to free the wrong
amount due to an assertion failure.

src/compile.cpp

@@ -7307,6 +7307,9 @@ finish(MyThread* t, FixedAllocator* allocator, Context* context)
        FixedSizeOfArray + ((context->objectPoolCount + 1) * BytesPerWord),
        true);
 
+    context->executableSize = (allocator->base + allocator->offset)
+      - static_cast<uint8_t*>(context->executableStart);
+
     initArray(t, pool, context->objectPoolCount + 1);
     mark(t, pool, 0);