C++: add false positives to MissingCheckScanf test

See github#12412 for the initial
report.

Author: redsun82

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

@@ -19,3 +19,5 @@
 | test.cpp:302:8:302:12 | ptr_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:301:3:301:7 | call to scanf | call to scanf |
 | test.cpp:310:7:310:7 | 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:309:3:309:7 | call to scanf | call to scanf |
 | 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 |

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

@@ -406,3 +406,20 @@ char *my_string_copy() {
 	*ptr++ = 0;
 	return DST_STRING;
 }
+
+void scan_and_write() {
+	{
+		int i;
+		if (scanf("%d", &i) < 1) {
+			i = 0;
+		}
+		use(i);  // GOOD [FALSE POSITIVE]: variable is overwritten with a default value when scanf fails
+	}
+	{
+		int i;
+		if (scanf("%d", &i) != 1) {
+			i = 0;
+		}
+		use(i);  // GOOD [FALSE POSITIVE]: variable is overwritten with a default value when scanf fails
+	}
+}