fix css regexp optimization

if you give "border: 1px solid rgb(0,0,0)" to the js css sanitizer,
it stripes out the rgb value, because of a bug in our regexp
optimizer.

the optimizer turns /foo{0}/ into /foo/ rather than //
so the pattern /(rgb...)(rgb...){0}/ becomes /(rgb...)(rgb...)/
which is the wrong thing to match.

this fixes https://code.google.com/p/google-caja/issues/detail?id=1517

[metaweta, 2012-09-05 00:21:17]

lgtm

[felix8a, 2012-09-05 00:31:14]

@r5036