Fix assertion in RegExp parser to correctly expect stack overflow.

Advance() always checks for stack overflow. If stack indeed overflowed,
current() would hold the kEndMarker. ParseOctalLiteral does not expect
this in the assertion, which causes assertion failure.

R=mvstanton@chromium.org
BUG=350865
LOG=N

Review URL: https://codereview.chromium.org/192773002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19764 ce2b1a6d-e550-0410-aec6-3dcde31c8c00