flush_rewrite_rules() should check $wp_rewrite->flush_rules() is callable before calling it

[markjaquith]

I ran into this situation:

Uncaught Error: Call to a member function flush_rules() on null

when some code was running flush_rewrite_rules() on shutdown.

flush_rewrite_rules() should do a sanity check before calling, so it never causes a fatal error.

[SergeyBiryukov]

In 45699:

Rewrite Rules: Check if $wp_rewrite->flush_rules() is callable before calling it in flush_rewrite_rules().

Props bsetiawan88, markjaquith.
Fixes #47087.