AddressSanitizer: Properly handle dispatch_source_set_cancel_handler with a
NULL handler

Per
https://developer.apple.com/library/mac/documentation/Performance/Reference/GCD_libdispatch_Ref/index.html,
the dispatch_source_set_cancel_handler() API *can* be called with a NULL
handler. In that case, the libdispatch removes an already existing cancellation
handler, if there was one. ASan's interceptor always creates a new block that
always tries to call the original handler. In case the original block is NULL,
a segmentation fault happens. Let's fix that by not wrapping a NULL-block at
all.

It looks like all the other libdispatch APIs (which we intercept) do *not*
allow NULL. So it's really only the dispatch_source_set_cancel_handler one that
needs this fix.

Reviewed at http://reviews.llvm.org/D6747