Fix: Properly extend the stack in ModifyArgInjector. (#144)

The old logic doesn't really make any sense and the calculated extension usually ended up being negative.
Instead, we just make room for our possible receiver and all of our args.

src/main/java/org/spongepowered/asm/mixin/injection/invoke/ModifyArgInjector.java

@@ -123,7 +123,12 @@ protected void injectAtInvoke(Target target, InjectionNode node) {
         }
 
         target.insns.insertBefore(methodNode, insns);
-        target.extendStack().set(2 - (extraLocals.get() - 1)).apply();
+        Extension extraStack = target.extendStack();
+        if (!isStatic) {
+            extraStack.add();
+        }
+        extraStack.add(methodArgs);
+        extraStack.apply();
         extraLocals.apply();
     }