add missing stdlib.h include
removed unnecessary stdlib.h include
fix table header indentation
Merge pull request #355 from mehlis/stdlib-fix

fix includes of stdlib
Merge pull request #369 from LudwigOrtmann/ps_fix_indent

fix table header indentation
catch some null pointers in RPL
provide a default next hop (preferred parent)
unlock mutexes when returning in an error case
Merge branch 'rpl_null_pointer' into safest_demo