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 bc487ae (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 <[email protected]>
[[email protected] added {} around the macro expansion]
Signed-off-by: Ben Pfaff <[email protected]>

lib/perf-counter.h

@@ -113,7 +113,7 @@ char *perf_counters_to_string(void);
           perf_counter_accumulate(&c, start_count);               \
       }
 #else
-#define PERF(name, expr)
+#define PERF(name, expr) { expr; }
 
 static inline void perf_counters_init(void) {}
 static inline void perf_counters_destroy(void) {}