Fix incorrect parameter offset calculation on Windows beyond argument 5

Fixes #17.

src/codegen.in.c

@@ -2113,7 +2113,11 @@ static void assign_lvar_offsets(Obj* prog) {
 
       var->offset = top;
       // outaf("int stack %s at stack 0x%x\n", var->name, var->offset);
-      top += MAX(8, var->ty->size);
+      if (var->is_param_passed_by_reference) {
+        top += 8;
+      } else {
+        top += MAX(8, var->ty->size);
+      }
     }
 
     // Assign offsets to local variables.

test/struct_bug17.c

@@ -0,0 +1,30 @@
+#include "test.h"
+
+struct StructA {
+  int x1, x2, x3;
+};
+
+void func(int dummy1,
+          int dummy2,
+          int dummy3,
+          int dummy4,
+          struct StructA trigger,
+          struct StructA bug_occurred) {
+  // Crash was occurring due to incorrect calculation of parameter offsets when
+  // passed by reference beyond register slots.
+  ASSERT(0, bug_occurred.x1);
+  ASSERT(1, bug_occurred.x2);
+  ASSERT(2, bug_occurred.x3);
+}
+
+extern void XXXXX(int dummy1,
+          int dummy2,
+          int dummy3,
+          int dummy4,
+          struct StructA trigger,
+          struct StructA bug_occurred);
+int main(void) {
+  struct StructA var = {0, 1, 2};
+  func(1, 2, 3, 4, var, var);
+  return 0;
+}