Make compiling a no-op if it's attempted before adding any patterns.

Fixes #485.

Change-Id: Id7900ab2d034432016d425aeceb60a814225f10e
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62872
Reviewed-by: Paul Wankadia <junyer@google.com>
Reviewed-by: Alex Chernyakhovsky <achernya@google.com>