Add null-checks in `XMLTest()`

When either `expected` or `found` is `NULL`, `XMLTest()` will segfault on `strcmp()`.
This patch adds null-checks, and passes the test if both `expected` and `found` are `NULL`.
diff --git a/xmltest.cpp b/xmltest.cpp
index 4325cb7..edd2e70 100644
--- a/xmltest.cpp
+++ b/xmltest.cpp
@@ -30,7 +30,12 @@
 

 bool XMLTest (const char* testString, const char* expected, const char* found, bool echo=true, bool extraNL=false )

 {

-	bool pass = !strcmp( expected, found );

+	bool pass;

+	if ( !expected && !found )

+		pass = true;

+	else if ( !expected || !found )

+		pass = false;

+	else pass = !strcmp( expected, found );

 	if ( pass )

 		printf ("[pass]");

 	else