Don't use stack to save/restore module on import.

Using the stack to save/restore the module pointer when doing things
like import would fail when an early exit would happen. As the jam stack
does not unwind, but instead is unwound/cleaned-up at exit later. And
the value to restore and check would not be the saved module. Instead
we save the module locally and restore that directly.

src/engine/frames.h

@@ -103,6 +103,7 @@ struct module_scope_in_function
 
 	private:
 	FRAME * module_frame = nullptr;
+    module_t * saved_module = nullptr;
 };
 
 }} // namespace b2::jam

src/engine/function.cpp

@@ -5380,17 +5380,15 @@ b2::jam::module_scope_in_function::module_scope_in_function(
     FRAME * frame_, const char * module_name_)
     : module_frame(frame_)
 {
-    auto stack = stack_global();
-    stack->push(b2::ensure_valid(module_frame->module));
+    saved_module = b2::ensure_valid(module_frame->module);
     module_frame->module = module_name_ == nullptr
         ? b2::ensure_valid(root_module())
         : b2::ensure_valid(bindmodule(value_ref(module_name_)));
 }
 
 b2::jam::module_scope_in_function::~module_scope_in_function()
 {
-    auto stack = stack_global();
-    module_frame->module = b2::ensure_valid(stack->pop<module_t *>());
+    module_frame->module = b2::ensure_valid(saved_module);
 }
 
 std::string b2::jam::backtrace::to_string(frame * f)