linux: Fix process start time computation
linux: Fix process start time computation

The process start time in ticks was being converted to an integer from a
temporary string that had gone out of scope by the time the conversion
was performed.

It was possible for a format error in /proc/pid/stat to go undetected
and result in a buffer overflow.

Bug: crashpad:30
Change-Id: I03566dda797bc1f23543bfffcfdb2c5ffe1eca66
Reviewed-on: https://chromium-review.googlesource.com/455378
Reviewed-by: Joshua Peraza <f5809a5e35ec7e1bd0e25cb0a0b68bb433cf6f17@chromium.org>
Commit-Queue: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>

chromium/crashpad,chromium/crashpad,chromium/crashpad,atom/crashpad,atom/crashpad,atom/crashpad