Fixes to chkprintcap:

• Check the return from a call to malloc() in skim_printcap(), and return a NULL if that fails.
• Fix a small memory leak in main() that happens if skim_printcap() returns an error, including the new error-return of NULL.

Submitted by: Tom Rix <trix@juniper.net>
Reviewed by: pfg, ngie
MFC after: 4 weeks
Sponsored by: Dell EMC Isilon, Juniper
Differential Revision: D9954, D9982