Fixed segfault in freeMemoryIfNeeded due to the fact that keys are now sds...

Fixed segfault in freeMemoryIfNeeded due to the fact that keys are now sds strings and not objects in the main hash table, thanks to Anthony Lauzon for spotting the bug and providing a patch.

src/redis.c

@@ -1321,7 +1321,8 @@ void freeMemoryIfNeeded(void) {
         if (tryFreeOneObjectFromFreelist() == REDIS_OK) continue;
         for (j = 0; j < server.dbnum; j++) {
             int minttl = -1;
-            robj *minkey = NULL;
+            sds minkey = NULL;
+            robj *keyobj = NULL;
             struct dictEntry *de;
 
             if (dictSize(server.db[j].expires)) {
@@ -1338,7 +1339,9 @@ void freeMemoryIfNeeded(void) {
                         minttl = t;
                     }
                 }
-                dbDelete(server.db+j,minkey);
+                keyobj = createStringObject(minkey,sdslen(minkey));
+                dbDelete(server.db+j,keyobj);
+                decrRefCount(keyobj);
             }
         }
         if (!freed) return; /* nothing to free... */