Initialise _v to 0 in typecheck case

This avoids using _v uninitialised if there's an empty typecheck
typemap, such as the dummy one we have for std::initializer_list.

Fixes GCC warning on cpp11_initializer_list testcase when compiled
with -O2.

CHANGES.current

@@ -7,6 +7,11 @@ the issue number to the end of the URL: https://github.com/swig/swig/issues/
 Version 4.1.0 (in progress)
 ===========================
 
+2021-04-19: olly
+	    Fix use of uninitialised variable in the generated code for an
+	    empty typecheck typemap, such as the dummy one we include for
+	    std::initializer_list.
+
 2021-04-12: olly
 	    #1777 [Python] Specifying -py3 now generates a check for Python
 	    version >= 3.0.

Source/Modules/overload.cxx

@@ -809,7 +809,7 @@ String *Swig_overload_dispatch(Node *n, const_String_or_char_ptr fmt, int *maxar
     }
 
     if (num_arguments) {
-      Printf(f, "int _v;\n");
+      Printf(f, "int _v = 0;\n");
     }
 
     int num_braces = 0;