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=plind44@gmail.com, yangguo@chromium.org
TEST=test/mjsunit/regress/regress-320948.js
BUG=chromium:320948
LOG=Y

Review URL: https://codereview.chromium.org/68793008

Patch from Balazs Kilvady <kilvadyb@homejinni.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@17891 ce2b1a6d-e550-0410-aec6-3dcde31c8c00