ubsan: Don’t call v[0] on empty vectors

ubsan: Don’t call v[0] on empty vectors

Calling std::vector<>::operator[]() with an out-of-range index argument
is undefined behavior. In two cases, Crashpad used &v[0] in situations
where it was known that the address would not be used. These calls were
wrapped in conditions guarding against vector emptiness.

While s[0] is valid on an empty string, in two cases, Crashpad used
&s[0] as an argument to a system call that would be a no-op. These calls
were wrapped in similar conditions to avoid the system call.

The two uses of vector with undefined behavior were caught by the
following tests in crashpad_snapshot_test with
UndefinedBehaviorSanitizer:

[ RUN      ] CrashpadInfoClientOptions.OneModule
/Users/mark/compilatorium/llvm.build/bin/../include/c++/v1/vector:1493:12:
runtime error: reference binding to null pointer of type
'crashpad::process_types::section'
[       OK ] CrashpadInfoClientOptions.OneModule (72 ms)

[ RUN      ] ProcessSnapshotMinidump.Empty
/Users/mark/compilatorium/llvm.build/bin/../include/c++/v1/vector:1493:12:
runtime error: reference binding to null pointer of type
'MINIDUMP_DIRECTORY'
[       OK ] ProcessSnapshotMinidump.Empty (1 ms)

The Crashpad codebase was audited by searching for resize() calls and
analyzing how resized strings and vectors are used.

TEST=*
BUG=
R=rsesek@chromium.org

Committed: https://chromium.googlesource.com/crashpad/crashpad/+/34aef02cc725795de2882e3ea30b048568048e12

[Mark Mentovai, 2015-08-20 15:14:15]

mark@chromium.org changed reviewers:
+ rsesek@chromium.org

[Robert Sesek, 2015-08-20 15:16:31]

LGTM.

Also, the CL description says "std::vector::operator<>() " -- should probably be
"std::vector<>::operator[]()"

[Mark Mentovai, 2015-08-20 15:50:09]

Thanks, fixed the description.

[Mark Mentovai, 2015-08-20 15:50:30]

Committed patchset #1 (id:1) manually as
34aef02cc725795de2882e3ea30b048568048e12 (presubmit successful).