top: avoid a potential SEGV during program termination

top: avoid a potential SEGV during program termination

The backtrace shown in the bug report referenced below
illustrates a 'normal' program termination interrupted
with some signal, ultimately then causing a top crash.

So this commit just rearranges a little code such that
all signals will be blocked during that rather lengthy
end of program processing regardless of how initiated.

[ in that report, ignore the assertion regarding the ]
[ '-n' option. it obviously was not '1' since do_key ]
[ had been called, which otherwise wouldn't be true. ]

[ and when it is '1' the -d option would be ignored. ]

Reference(s):
https://bugzilla.redhat.com/show_bug.cgi?id=1737552

Signed-off-by: Jim Warner <james.warner@comcast.net>