Trap on rescan following rename caused by pci->pszFileName being NullStr

[Gregg Young]

After I "fixed" the direct edit code it would leave a blank directory line above or below the renamed directory. I added a rescan and the problem was covered up. This seemed fine until it trapped. I traced the trap to UM_RESCAN2 treecnr.c ~791

if (pci && (INT) pci != -1 && pci->pszFileName != NullStr) { //fixme? will try checking pci->pszFileName instead of the pointer
	  if (!(driveflags[toupper(*pci->pszFileName) - 'A'] &
		DRIVE_REMOVABLE) ||
	      driveserial[toupper(*pci->pszFileName) - 'A'] != -1) {

I added the != NullStr which fixed the trap. I can see we had problems with this in the past based on the fixme. Hopefully this fixed that problem also.