GDScript: Don't use the NIL address to hold return value of functions

This prevents that the NIL address is filled with another value, which
causes problems for some instructions that read from NIL.

modules/gdscript/tests/scripts/runtime/features/standalone-calls-do-not-write-to-nil.gd

@@ -0,0 +1,45 @@
+# https://github.com/godotengine/godot/issues/70964
+
+func test():
+	test_construct(0, false)
+	test_utility(0, false)
+	test_builtin_call(Vector2.UP, false)
+	test_builtin_call_validated(Vector2.UP, false)
+	test_object_call(RefCounted.new(), false)
+	test_object_call_method_bind(Resource.new(), false)
+	test_object_call_ptrcall(RefCounted.new(), false)
+
+	print("end")
+
+func test_construct(v, f):
+	Vector2(v, v) # Built-in type construct.
+	assert(not f) # Test unary operator reading from `nil`.
+
+func test_utility(v, f):
+	abs(v) # Utility function.
+	assert(not f) # Test unary operator reading from `nil`.
+
+func test_builtin_call(v, f):
+	@warning_ignore(unsafe_method_access)
+	v.angle() # Built-in method call.
+	assert(not f) # Test unary operator reading from `nil`.
+
+func test_builtin_call_validated(v: Vector2, f):
+	@warning_ignore(return_value_discarded)
+	v.abs() # Built-in method call validated.
+	assert(not f) # Test unary operator reading from `nil`.
+
+func test_object_call(v, f):
+	@warning_ignore(unsafe_method_access)
+	v.get_reference_count() # Native type method call.
+	assert(not f) # Test unary operator reading from `nil`.
+
+func test_object_call_method_bind(v: Resource, f):
+	@warning_ignore(return_value_discarded)
+	v.duplicate() # Native type method call with MethodBind.
+	assert(not f) # Test unary operator reading from `nil`.
+
+func test_object_call_ptrcall(v: RefCounted, f):
+	@warning_ignore(return_value_discarded)
+	v.get_reference_count() # Native type method call with ptrcall.
+	assert(not f) # Test unary operator reading from `nil`.

modules/gdscript/tests/scripts/runtime/features/standalone-calls-do-not-write-to-nil.out

@@ -0,0 +1,2 @@
+GDTEST_OK
+end