procrank/procmem fail on 0-length page mappings

procrank prints messages such as:
warning: could not read usage for 279
and does not include that pid in its table of results

procmem segfaults when run against that process

The problem is the 0-length page map at the top of the stack:

beb39000-beb39000 rw-p 00000000 00:00 0
beb39000-beb5a000 rw-p 00000000 00:00 0          [stack]

This patch changes the code to skip over that 0-length mapping when calculating
memory usage

Change-Id: I02aaaecdf64affbd28ab43a2d0865edda33816bc