BUG/MINOR: tune.fail-alloc: Don't forget to initialize ret.

In mem_should_fail(), if we don't want to fail the allocation, either
because mem_fail_rate is 0, or because we're still initializing, don't
forget to initialize ret, or we may return a non-zero value, and fail
an allocation we didn't want to fail.
This should only affect users that compile with DEBUG_FAIL_ALLOC.
diff --git a/src/memory.c b/src/memory.c
index 04259a5..b200c38 100644
--- a/src/memory.c
+++ b/src/memory.c
@@ -581,7 +581,7 @@
 
 int mem_should_fail(const struct pool_head *pool)
 {
-	int ret;
+	int ret = 0;
 	int n;
 
 	if (mem_fail_rate > 0 && !(global.mode & MODE_STARTING)) {