Fix ast_(v)asprintf() malloc failure usage conditions.

When (v)asprintf() fails, the state of the allocated buffer is undefined.
The library had better not leave an allocated buffer as a result or no one
will know to free it.  The most likely way it can return failure is for an
allocation failure.  If the printf conversion fails then you actually have
a threading problem which is much worse because another thread modified
the parameter values.

* Made __ast_asprintf()/__ast_vasprintf() set the returned buffer to NULL
on failure.  That is much more useful than either an uninitialized pointer
or a pointer that has already been freed.  Many uses won't have to check
for failure to ensure that the buffer won't be double freed or prevent an
attempt to free an uninitialized pointer.

* stasis.c: Fixed memory leak in multi_object_blob_to_ami() allocated by
ast_asprintf().

* ari/resource_bridges.c:ari_bridges_play_helper(): Remove assignment to
the wrong thing which is now not needed even if assigning to the right
thing.

Change-Id: Ib5252fb8850ecf0f78ed0ee2ca0796bda7e91c23