MIPS: Fixed crashes exposed though fuzzing.

MIPS: Fixed crashes exposed though fuzzing.

Port r17886 (e2fb3ed)

Original commit message:
The %_OneByteSeqStringSetChar intrinsic expects its arguments to be checked before being called for efficiency reasons, but the fuzzer provided no such checks. Now the intrinsic is robust to bad input if FLAG_debug_code is set.

R=yangguo@chromium.org
TEST=test/mjsunit/regress/regress-320948.js
BUG=chromium:320948
LOG=Y

[Paul Lind, 2013-11-19 22:18:50]

I do have some nits on this, but since this is a build-breaker for MIPS, we'll
take those issues offline, and submit a 2nd CL if needed.

LGTM.

[Paul Lind, 2013-11-19 22:25:09]

Committed as r17891.