C++: add further FP to test

cpp/ql/test/query-tests/Critical/MissingCheckScanf/MissingCheckScanf.expected

@@ -21,3 +21,4 @@
 | test.cpp:404:25:404:25 | u | This variable is read, but may not have been written. It should be guarded by a check that the $@ returns at least 1. | test.cpp:403:6:403:11 | call to sscanf | call to sscanf |
 | test.cpp:416:7:416:7 | i | This variable is read, but may not have been written. It should be guarded by a check that the $@ returns at least 1. | test.cpp:413:7:413:11 | call to scanf | call to scanf |
 | test.cpp:423:7:423:7 | i | This variable is read, but may not have been written. It should be guarded by a check that the $@ returns at least 1. | test.cpp:420:7:420:11 | call to scanf | call to scanf |
+| test.cpp:430:6:430:6 | i | This variable is read, but may not have been written. It should be guarded by a check that the $@ returns at least 1. | test.cpp:429:2:429:6 | call to scanf | call to scanf |

cpp/ql/test/query-tests/Critical/MissingCheckScanf/test.cpp

@@ -423,3 +423,9 @@ void scan_and_write() {
 		use(i);  // GOOD [FALSE POSITIVE]: variable is overwritten with a default value when scanf fails
 	}
 }
+
+void scan_and_static_variable() {
+	static int i;
+	scanf("%d", &i);
+	use(i);  // GOOD [FALSE POSITIVE]: static variables are always 0-initialized
+}