Fixed #4512 (Improve check: Uninitialized variable not detected)

lib/checkuninitvar.cpp

@@ -1498,6 +1498,10 @@ bool CheckUninitVar::isVariableUsage(const Scope* scope, const Token *vartok, bo
                     if ((pointer || address) && Token::Match(argStart, "const %type% * %var% [,)]"))
                         return true;
                 }
+
+            } else if (Token::Match(start->previous(), "if|while|for")) {
+                // control-flow statement reading the variable "by value"
+                return true;
             }
         }
     }

test/testuninitvar.cpp

@@ -2010,6 +2010,12 @@ private:
         ASSERT_EQUALS("", errout.str());
 
         // using uninit var in condition
+        checkUninitVar2("void f(void) {\n"
+                        "    int x;\n"
+                        "    if (x) { }\n"
+                        "}");
+        ASSERT_EQUALS("[test.cpp:3]: (error) Uninitialized variable: x\n", errout.str());
+
         checkUninitVar2("void f() {\n"
                         "    int x;\n"
                         "    if (1 == (3 & x)) { }\n"