MathLib: Added missing testcase for incdec.

lib/mathlib.cpp

@@ -548,7 +548,6 @@ std::string MathLib::incdec(const std::string & var, const std::string & op)
 
     throw InternalError(0, std::string("Unexpected operation '") + op + "' in MathLib::incdec(). Please report this to Cppcheck developers.");
     return "";
-
 }
 
 std::string MathLib::divide(const std::string &first, const std::string &second)

test/testmathlib.cpp

@@ -800,6 +800,8 @@ private:
             const MathLib::biguint decrementedNum = MathLib::toULongNumber(strNum);
             ASSERT_EQUALS(num - 1U, decrementedNum);
         }
+        // invalid operation
+        ASSERT_THROW(MathLib::incdec("1", "x"), InternalError); // throw
     }
 };