Travis: Fix one more 'nonneg' issue

.travis.yml

@@ -208,10 +208,8 @@ script:
   - echo $CXXFLAGS
   - make -s test -j2 CPPFLAGS=-DTEST_MATHLIB_VALUE
   - touch lib/mathlib.cpp test/testmathlib.cpp
-# compile cppcheck with -DNONNEG
-  - echo $CXXFLAGS
-  - make -s CPPFLAGS=-DNONNEG -j2
-  - make clean
+# syntax checking of cppcheck source code with -DNONNEG
+  - g++ -fsyntax-only -Ilib -Iexternals -Iexternals/simplecpp -Iexternals/tinyxml -DNONNEG lib/*.cpp cli/*.cpp
 # compile cppcheck, default build
   - echo $CXXFLAGS
   - make -s test -j2

lib/tokenize.cpp

@@ -7221,7 +7221,7 @@ bool Tokenizer::simplifyKnownVariables()
                 const Token * const valueToken = tok2->tokAt(2);
 
                 std::string value;
-                int valueVarId = 0;
+                nonneg int valueVarId = 0;
 
                 Token *tok3 = nullptr;
                 bool valueIsPointer = false;