test: logalloc_test: Fix build for boost 1.63
test: logalloc_test: Fix build for boost 1.63

Due to https://svn.boost.org/trac10/ticket/12778?replyto=3
BOOST_REQUIRE_NE does not work with nullptr.

Tests: units (release)

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>
Message-Id: <e7158e8a235356fad99560f6fcbecb57615cefe6.1523298193.git.2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

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