top: protect against the anomalous 'Mem' graph display

Until this patch, top falsely assumed that there would
always be some (small) amount of physical memory after
subtracting 'used' and 'available' from the total. But
as the issue referenced below attests, a sum of 'used'
and 'available' might exceed that total memory amount.

I'm not sure if this is a problem with our calculation
of the 'used' amount, a flaw in the kernel 'available'
algorithms or some other reason I cannot even imagine.

Anyway, this patch protects against such a contingency
through the following single line addition of new code
. if (pct_used + pct_misc > 100.0 || pct_misc < 0) ...

The check for less than zero is not actually necessary
as long as the source numbers remain unsigned. However
should they ever become signed, we'll have protection.

[ Most of the changes in this commit simply separate ]
[ a variable's definition from its associated logic. ]

Reference(s):
#64



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