Paths config doesn't validate on close

[YoshiRulz]

Should maybe validate at the top of this loop:

BizHawk/src/BizHawk.Client.EmuHawk/config/PathConfig.cs

Lines 218 to 222 in f28fa49

	foreach (var t in AllPathControls.OfType<TextBox>())
	{
	var pathEntry = _pathEntries.Paths.First(p => p.System == t.Parent.Name && p.Type == t.Name);
	pathEntry.Path = t.Text;
	}

Initially reported by @Spikestuff on Discord.

see also #1574

[Morilli]

Perhaps it should also explicitly allow the case that caused this report; having an otherwise valid path enclosed in "".