Fix bugs found by Coverity in decryptcore(8) and savecore(8):

Fix bugs found by Coverity in decryptcore(8) and savecore(8):
- Perform final decryption and write decrypted data in case of non-block aligned
input data;
- Use strlcpy(3) instead of strncpy(3) to verify if paths aren't too long;
- Check errno after calling unlink(2) instead of calling stat(2) in order to
verify if a decrypted core was created by a child process;
- Free dumpkey.

Reported by:	Coverity, cem, pfg
Suggested by:	cem
CID:		1366936, 1366942, 1366951, 1366952
Approved by:	pjd (mentor)