dist/cppcheck: handle unhandled condition

When no arguments are given at all, the script would shift when there
is nothing to shift. This results in a failure on debian stable:
```
./dist/tools/cppcheck/check.sh: 20: shift: can't shift that many
```
Merge pull request #2069 from LudwigOrtmann/cppcheck-fixup

dist/cppcheck: handle unhandled condition