VTE: Fix crash when "send_cmd_prefix" is NULL

[eht16]

Related to #3151 and #3149.

The NULL check in vte_send_cmd() is just for feeling more safe, the main change is to consider vc->send_cmd_prefix being NULL when constructing the command to be executed when we want to change the directory in the VTE.
This should not happen but it might.

[kugel-]

Obsoleted by #3156 (IMO)

[frlan]

Having this patch active I didn't see further crashes hitting e.g. f5 most likely caused by in thix context unset send_cmd_prefix

[eht16]

Obsoleted by #3156 (IMO)

Not necessarily I'd say.
I think the guards here are useful anyway. True, it should not happen that vte_config.send_cmd_prefix is NULL and that any caller should pass NULL to vte_send_cmd but still it might happen and Geany should not crash then.

[kugel-]

But we aren't going to augment every pref in case garbage is read from geany.conf, do we?

[eht16]

No, of course not.
But since we now know that vte_config.send_cmd_prefix might lead to crashes when it is NULL for whatever reasons, checking it makes it not worse, IMO.

[kugel-]

I'm not blocking, go ahead if you think it's worthwhile

[elextr]

@eht16 ping, still complaints about this problem, see #3190

[eht16]

Just fixed the merge conflict. If there are no objections, let's merge it.

[elextr]

LGTM