Fix: ust-consumerd: leak of stream control structure

Fix: ust-consumerd: leak of stream control structure

The following leak is reported by LeakSanitizer when
setup_metadata() fails to send the metadata stream to the relay
daemon:

  ==3050181==ERROR: LeakSanitizer: detected memory leaks

  Direct leak of 240 byte(s) in 5 object(s) allocated from:
      #0 0x7f5fce02cfb9 in __interceptor_calloc /usr/src/debug/gcc/libsanitizer/asan/asan_malloc_linux.cpp:154
      #1 0x7f5fcdd95a7a in zmalloc ../../../src/common/macros.h:23
      #2 0x7f5fcdd95a7a in lttng_ust_ctl_create_stream /home/jgalar/EfficiOS/src/lttng-ust/src/lib/lttng-ust-ctl/ustctl.c:1649

A consumer stream can have an allocated
`struct lttng_ust_ctl_consumer_stream *` (ustream) even if it is
not globally visible at the time of its teardown.

In the case of the user space consumer, the only site that creates
consumer stream instances ensures that the allocation of the
lttng_ust_ctl_consumer_stream succeeded, ensuring that the
consumer stream's 'ustream' is always set.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ia4be7969e85bd8812ae13b042e1e100812a63c1d