C++: Add testcase demonstrating that the model for 'strncpy' is broken.

cpp/ql/test/library-tests/dataflow/taint-tests/taint.cpp

@@ -702,4 +702,12 @@ namespace strings {
 		*p++ = *s;
 		sink(p); // $ ast ir
 	}
+}
+
+char * strncpy (char *, const char *, unsigned long);
+
+void test_strncpy(char* d, char* s) {
+	argument_source(s);
+	strncpy(d, s, 16);
+	sink(d); // $ ast MISSING: ir
 }