box: check return value of obuf_alloc() in xlog_tx_write_zstd()

obuf_alloc(&log->zbuf, XLOG_FIXHEADER_SIZE) can potentially fail,
because there is no obuf_reserve() prior to it.

Closes tarantool/security#74

NO_DOC=bugfix
NO_CHANGELOG=bugfix
NO_TEST=no test harness for checking OOM

(cherry picked from commit 32dfcb3c)