SR-4677: avoid SIGSEGV if _dispatch_muxnote_create returns NULL
SR-4677: avoid SIGSEGV if _dispatch_muxnote_create returns NULL

It is possible for _dispatch_muxnote_create to return NULL, this error
condition was not being handled in dispatch_unote_register leading to
an immediate SIGSEGV when it happened.

Signed-off-by: Daniel A. Steffen <bc823475b60dbcbd7be1bbdfe095b5f1939d5bd2@apple.com>

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch