fix static analysis and compiler warnings

The clang scan-build tools proposed to fix this potential security issue.
Same tool told to a default case to ensure variable p is always set
before calling inet_ntop() in construct_pid_file().

The gcc with all nagging turned on warned about the following, and these
got to be fixed as well.

f5gs.c:119:11: warning: ignoring return value of 'vasprintf', declared
with attribute warn_unused_result [-Wunused-result]
f5gs.c:315:7: warning: ignoring return value of 'chdir', declared with
attribute warn_unused_result [-Wunused-result]
f5gs.c:547:8: warning: ignoring return value of 'fscanf', declared with
attribute warn_unused_result [-Wunused-result]
f5gs.c:535:6: warning: ignoring return value of 'read', declared with
attribute warn_unused_result [-Wunused-result]

Signed-off-by: Sami Kerola <kerolasa@iki.fi>