Merge r1065: Don't bail if a thread's stack pointer is invalid

Currently, if a thread's stack pointer is not within a valid memory page,
the minidump writing will fail with an error.  This change allows an invalid
stack pointer by simply setting the memory size to zero in the minidump.
The processing code already checks for the size being zero, although it
currently just gives an error (see

BUG=google-breakpad:499, chromium-os:34880
TEST=make check, manually ran minidump-2-core and core2md
Original Review URL:

* To make this build, there is an additional #include <ucontext.h>.
Review URL:

git-svn-id: 4c0a9323-5329-0410-9bdc-e9ce6186880e
7 files changed