top: fix additional SEGVs if no tasks were displayable

top: fix additional SEGVs if no tasks were displayable

This patch is an outgrowth of that commit shown below.

Many additional potential segmentation faults might be
encountered if interactive commands are opened up to a
user when a '-p' switch has a single non-existent pid.

[ always the 'k', 'L', 'r', 'Y' keys & maybe 'v' too ]

So, this patch will restrict such a loser (oops, user)
to a reduced subset of normal commands until he/she/it
quits then restarts top with something to be displayed
or issues the '=' command overriding that '-p' switch.

Reference(s):
commit f57a0301e3adfa5fd456404a200182c7f21da03a

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