perf-counter: Fix segfaults on non-Linux platforms.

perf-counter: Fix segfaults on non-Linux platforms.

Commit 0eee08dbddea520536197657da7a0b (perf-counter: simplify the
performance macro) introduces the PERF macro, which is empty for non-linux
platforms.

Added to commit bc487aeff2d6823c088d6e4499e0f53e6651523b (ovsdb: Add per
transaction commit instruction counter), that uses such macro, it causes
segfaults, since it makes ovsdb_txn_commit return an uninitialized pointer as
error.

Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@redhat.com>
[blp@nicira.com added {} around the macro expansion]
Signed-off-by: Ben Pfaff <blp@nicira.com>