Fix #14171 FP leakReturnValNotUsed with freopen() (danmar#7866)

Co-authored-by: chrchr-github <noreply@github.com>

Author: chrchr-github

lib/checkmemoryleak.cpp

@@ -1117,6 +1117,8 @@ void CheckMemoryLeakNoVar::checkForUnusedReturnValue(const Scope *scope)
                 if (!(Token::simpleMatch(parent->astParent(), "?") && !parent->astParent()->astParent()))
                     continue;
             }
+            if (tok->str() == "freopen")
+                continue;
             returnValueNotUsedError(tok, tok->str());
         }
     }

test/testmemleak.cpp

@@ -2701,6 +2701,12 @@ class TestMemleakNoVar : public TestFixture {
               "    delete[] &p[-1];\n"
               "}\n");
         ASSERT_EQUALS("", errout_str());
+
+        check("void f(FILE* fp1, FILE* fp2) {\n" // #14171
+              "    if (freopen(NULL, \"w+b\", fp1) == NULL) {}\n"
+              "    if (std::freopen(NULL, \"w+b\", fp2) == NULL) {}\n"
+              "}\n");
+        ASSERT_EQUALS("", errout_str());
     }
 
     void smartPointerFunctionParam() {