Merge pull request #208 from ktopley-apple/dispatch-data-null-buffer

Fixes crash when DispatchData is created from an UnsafeBufferPointer<UInt8> with a nil address (Radar 29337927)

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

Author: moiseev

PATCHES

@@ -308,3 +308,4 @@ github commits starting with 29bdc2f from
 [b6e9cf4] APPLIED rdar://30568673
 [e199473] APPLIED rdar://30568673
 [3767ac7] APPLIED rdar://30568673
+[10eb0e4] APPLIED rdar://30568673