add an assertion to make sure numkeys is 0 in getKeysUsingKeySpecs (r…

…edis#12389)

This is an addition to redis#12380, to prevent potential bugs when collecting
keys from multiple commands in the future.
Note that this function also resets numkeys in some cases.

src/db.c

@@ -1873,7 +1873,7 @@ int64_t getAllKeySpecsFlags(struct redisCommand *cmd, int inv) {
 int getKeysUsingKeySpecs(struct redisCommand *cmd, robj **argv, int argc, int search_flags, getKeysResult *result) {
     int j, i, last, first, step;
     keyReference *keys;
-    result->numkeys = 0;
+    serverAssert(result->numkeys == 0); /* caller should initialize or reset it */
 
     for (j = 0; j < cmd->key_specs_num; j++) {
         keySpec *spec = cmd->key_specs + j;