Fix map-crash when floating npc use *npcspeed script command

[Emistry]

Pull Request Prelude

• I have followed proper Hercules code styling.
• I have read and understood the contribution guidelines before making this PR.
• I am aware that this PR may be closed if the above-mentioned criteria are not fulfilled.

Changes Proposed

Check for unit data (nd->ud != NULL) before adjust the NPC speed.
Throw warnings if nd->ud == NULL.

Issues addressed:
script command npcspeed, npcwalkto, npcstop, unitwalk, unitwarp, unitstop are not applicable for floating npc (without coordinate/location).
Fix map-crash due to unit_data NULL.

Sample NPC Script:

-	script	dummynpc	-1,{
	OnInit:
		debugmes "set speed to 100";
		npcspeed 100;
		debugmes "set speed done";
		end;
}

Result:

After fixed: throw warnings but didnt crash map-server

Related PR #1733

[HerculesWSAPI]

This change is 

[AnnieRuru]

there are similar lines in *npcwalkto and *npcstop
however when I test it, it doesn't crash server
so I guess its OK ... I guess ?

[4144]

@AnnieRuru this code may not crash server but corrupt memory

probably better fix it in all functions

[AnnieRuru]

at least it doesn't cause memory leak ...
well, if just add in the same lines in those 2 script commands, simple I think

[Emistry]

the reason it doesn't crash are probably due to :
npcwalkto() actually have the checking if( ud == NULL) return 0; added inside unit_walktoxy()
and
npcstop() actually have the checking if(!ud ..... ) return 0; added inside unit_stop_walking()

[AnnieRuru]

how about just adding

		if (nd->ud == NULL) {
			ShowWarning("buildin_npcwalkto: floating NPC don't have unit data.\n");
			return false;
		}

instead of let the script command fail silently ?

[hemagx]

Emistry, can you separate syntax fixing from the crash fixing? it's sort of impossible to review as it is right now.
separating each in it's respective commit would be a lot better.

[Emistry]

huh? like how? reset then redo to create 2 different commits?
aren't at the end it still gonna end up become 1 commit that fix the same thing?
beside its not a huge commit to begin with, most of it just the if (ud == NULL) checking.