Handle undefined WINDIR on POSIX platforms

The test for whether it was defined was failing with an error on
platforms where it wasn't defined.

ghidra/delink.sh

@@ -67,8 +67,8 @@ delink() {
 # $5: Output path (inside decompile/target/)
 	# Call the right script depending on whether we're on Windows
 	# (this seems like the most reliable method I can find for POSIX sh)
-	if [ -n "$WINDIR" ]; then suffix=.bat
-	else                      suffix=
+	if [ -n "${WINDIR-}" ]; then suffix=.bat
+	else                         suffix=
 	fi
 
 	# POSIX compatibility layers for Windows (like in git bash) will