scrotExecApp cleanup

[N-R-K]

scrotExecApp: fix memory leak and style cleanup

• execStr is allocated via imPrintf, so free it before returning.
• remove unnecessary exit(0) - simply let the control return back to main() where it will do some cleanup and exit anyways.
• remove useless errno = 0.
• initialize execStr and ret immediately.

scrotExecApp: check for WIFEXITED first

according to waitpid(3) manpage:

| This macro (WEXITSTATUS) should be employed only if WIFEXITED returned true.