[NaCl SDK] nacl_io: fix invalid access to string data.

[NaCl SDK] nacl_io: fix invalid access to string data.

The data referenced by c_str() is only valid as long as the
std::string is still live, so hold std::string values
on the stack.

This was causing tests to fail on my Trusty laptop.

BUG=
R=binji@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=285346

[binji, 2014-07-24 17:07:44]

lgtm, but why wasn't this failing anywhere else, I wonder?

[Sam Clegg, 2014-07-24 17:30:22]

The CQ bit was checked by sbc@chromium.org

[commit-bot: I haz the power, 2014-07-24 17:31:32]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/sbc@chromium.org/414843004/1

[commit-bot: I haz the power, 2014-07-24 18:24:10]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_chromium_rel_swarming on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_rel_sw...)

[commit-bot: I haz the power, 2014-07-24 19:03:33]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-07-24 19:03:34]

Try jobs failed on following builders:
  linux_chromium_rel_swarming on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/linux_chromium_rel_sw...)

[Sam Clegg, 2014-07-24 19:09:05]

Committed patchset #1 manually as r285346 (presubmit successful).