travis,tests: run cppcheck only if it exists

ugh... seems cppcheck is not packaged for OS X
And `set -e` exposes this.

And also `cppcheck` seems to exit with non-zero
exit codes by default [even if errs found].

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

.travis.yml

@@ -29,4 +29,4 @@ script:
 
 after_success:
   - make check
-  - cppcheck --quiet *.h *.c tests/
+  - if type cppcheck &> /dev/null ; then cppcheck --error-exitcode=1 --quiet *.h *.c tests/ ; fi