Prevent reentrant calling of ctkProxyStyle::ensureBaseStyle()

If ctkProxyStyle::ensureBaseStyle() is already in progress for an object then further ctkProxyStyle::ensureBaseStyle() calls for the same object are ignored.
This patch is tested and does prevent the crashes. There doesn't seem to be any side-effect.
Merge pull request #372 from lassoan/371-ensure-base-style-infinite-recursion-crash

Prevent reentrant calling of ctkProxyStyle::ensureBaseStyle()