Fixing memory safety issue on empty blocks.
Fixing memory safety issue on empty blocks.

From Facebook’s Infer:
TOCInternal_BlockObject.m:16: warning: IVAR_NOT_NULL_CHECKED
   Instance variable self -> block is not checked for null, there could
be a null pointer dereference: pointer self->block last accessed on
line 16 could be null and is dereferenced at line 16, column 5

Strilanc/ObjC-CollapsingFutures