CodeCache::m_capacity can becoming negative, producing undefined results in pruneSlowCase
https://bugs.webkit.org/show_bug.cgi?id=113453

Reviewed by Geoffrey Garen.

* runtime/CodeCache.cpp:
(JSC::CodeCacheMap::pruneSlowCase): We make sure that m_minCapacity doesn't drop below zero now.
This prevents m_capacity from doing the same.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@147017 268f45cc-cd09-0410-ab3c-d52691b4dbfc