Fix keytable size calculation

We only need to allocate memory for key positions, not
for every argument position.  The most keys we can have
is total arguments / keystep.

Closes redis#1300

src/db.c

@@ -992,7 +992,7 @@ int *getKeysUsingCommandTable(struct redisCommand *cmd,robj **argv, int argc, in
     }
     last = cmd->lastkey;
     if (last < 0) last = argc+last;
-    keys = zmalloc(sizeof(int)*((last - cmd->firstkey)+1));
+    keys = zmalloc(sizeof(int)*((last - cmd->firstkey)+1) / cmd->keystep);
     for (j = cmd->firstkey; j <= last; j += cmd->keystep) {
         redisAssert(j < argc);
         keys[i++] = j;