build: fix Werror warning in src/box/txn.c

Found on GCC 4.8.5 on CentOS 7 issue:

  build/usr/src/debug/tarantool-2.6.0.144/src/box/txn.c:944:30: error: ‘limbo_entry’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
     if (txn_limbo_wait_complete(&txn_limbo, limbo_entry) < 0)

Set limbo_entry variable to NULL on initialization.

Needed for #4941