rcl_ros_clock_init stores pointer to allocator rather than storing a copy

[wjwwood]

See: #264 (comment)

This line should be changed so that the given allocator is copied, either by allocating memory to store the allocator struct as a pointer in the rcl_clock_t or by changing the pointer to an allocator in the rcl_clock_t to an instance and then assigning the incoming allocator structure to the one in the rcl_clock_t:

rcl/rcl/src/rcl/time.c

Line 140 in d13a902

clock->allocator = allocator;