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/gdscript_byte_codegen.h

@@ -309,6 +309,8 @@ class GDScriptByteCodeGenerator : public GDScriptCodeGenerator {
 		}
 	}
 
+	Address get_call_target(const Address &p_target);
+
 	int address_of(const Address &p_address) {
 		switch (p_address.mode) {
 			case Address::SELF: