libpagemap: handle zero length memory regions

/proc/pid/maps may report a zero-length memory region for a 4kB
PROT_GROWSDOWN region because it subtracts 4kB for the guard page.
Return 0 instead of -1 when this occurs, and set range_out to NULL
and len to 0.  All existing callers of pm_process_pagemap_range
will not dereference range_out if len is 0.

Bug: 14683277
Change-Id: If405651ad034dda780b93fab2dc616e177a0b917
Merge "libpagemap: handle zero length memory regions"
auto-merger: merge in from aosp/master