unblockClient: avoid to reset client when the client was shutdown-blocked (#10440)

fix #10439. see https://github.com/redis/redis/pull/9872
When executing SHUTDOWN we pause the client so we can un-pause it
if the shutdown fails.
this could happen during the timeout, if the shutdown is aborted, but could
also happen from withing the initial `call()` to shutdown, if the rdb save fails.
in that case when we return to `call()`, we'll crash if `c->cmd` has been set to NULL.

The call stack is:
```
unblockClient(c)
replyToClientsBlockedOnShutdown()
cancelShutdown()
finishShutdown()
prepareForShutdown()
shutdownCommand()
```

what's special about SHUTDOWN in that respect is that it can be paused,
and then un-paused before the original `call()` returns.
tests where added for both failed shutdown, and a followup successful one.

src/blocked.c

@@ -204,7 +204,7 @@ void unblockClient(client *c) {
      * we do not do it immediately after the command returns (when the
      * client got blocked) in order to be still able to access the argument
      * vector from module callbacks and updateStatsOnUnblock. */
-    if (c->btype != BLOCKED_POSTPONE) {
+    if (c->btype != BLOCKED_POSTPONE && c->btype != BLOCKED_SHUTDOWN) {
         freeClientOriginalArgv(c);
         resetClient(c);
     }

tests/unit/shutdown.tcl

@@ -66,3 +66,39 @@ start_server {tags {"shutdown external:skip"}} {
         }
     }
 }
+
+start_server {tags {"shutdown external:skip"}} {
+    set pid [s process_id]
+    set dump_rdb [file join [lindex [r config get dir] 1] dump.rdb]
+
+    test {RDB save will be failed in shutdown} {
+        for {set i 0} {$i < 20} {incr i} {
+            r set $i $i
+        }
+
+        # create a folder called 'dump.rdb' to trigger temp-rdb rename failure
+        # and it will cause rdb save to fail eventually.
+        if {[file exists $dump_rdb]} {
+            exec rm -f $dump_rdb
+        }
+        exec mkdir -p $dump_rdb
+    }
+    test {SHUTDOWN will abort if rdb save failed on signal} {
+        # trigger a shutdown which will save an rdb
+        exec kill -SIGINT $pid
+        wait_for_log_messages 0 {"*Error trying to save the DB, can't exit*"} 0 100 10
+    }
+    test {SHUTDOWN will abort if rdb save failed on shutdown command} {
+        catch {[r shutdown]} err
+        assert_match {*Errors trying to SHUTDOWN*} $err
+        # make sure the server is still alive
+        assert_equal [r ping] {PONG}
+    }
+    test {SHUTDOWN can proceed if shutdown command was with nosave} {
+        catch {[r shutdown nosave]}
+        wait_for_log_messages 0 {"*ready to exit, bye bye*"} 0 100 10
+    }
+    test {Clean up rdb same named folder} {
+        exec rm -r $dump_rdb
+    }
+}