top: fix potential SEGV when no tasks were displayable

top: fix potential SEGV when no tasks were displayable

This patch fixes a nearly decade old bug discovered by
Frederik Deweerdt. His merge request shown below would
be an adequate solution except for iterative overhead.

This alternate patch will represent substantially less
overhead for an admittedly extremely rare possibility.

Reference(s):
https://gitlab.com/procps-ng/procps/-/merge_requests/114

And-thanks-to: Frederik Deweerdt <fdeweerdt@fastly.com>
Signed-off-by: Jim Warner <james.warner@comcast.net>