Add test for #12391

Author: chrchr-github

test/testcondition.cpp

@@ -5678,6 +5678,15 @@ class TestCondition : public TestFixture {
               "}\n");
         ASSERT_EQUALS("[test.cpp:2] -> [test.cpp:3]: (style) The if condition is the same as the previous if condition\n", errout_str());
 
+        check("struct S { int x; };\n" // #12391
+              "int f(const struct S* a, const struct S* b) {\n"
+              "    const struct S* p = b;\n"
+              "    if (a->x < p->x) p++;\n"
+              "    if (a->x < p->x) p++;\n"
+              "    return p->x;\n"
+              "}\n");
+        ASSERT_EQUALS("", errout_str());
+
         // do not crash
         check("void assign(const MMA& other) {\n"
               "    if (mPA.cols != other.mPA.cols || mPA.rows != other.mPA.rows)\n"