Never call MacroAssembler::Allocate() with incorrect size.

Never call MacroAssembler::Allocate() with incorrect size.

BUG=chromium:412206
LOG=n

[Igor Sheludko, 2014-10-10 10:24:13]

ishell@chromium.org changed reviewers:
+ jkummerow@chromium.org

[Igor Sheludko, 2014-10-10 10:24:14]

PTAL

[Jakob Kummerow, 2014-10-10 10:41:30]

LGTM.

I'm not too happy with this, because:
(1) I'm not convinced %_RegexpConstructResult needs to be fuzzer-proof.
(2) There's a much deeper issue at play here, namely widespread use of signed
int types for size values that should be size_t. But since this local fix is
several orders of magnitude more feasible than a proper cleanup, I guess that's
fine for now...