runtime(netrw): correctly handle shellslash variable

closes: #16758

Signed-off-by: Luca Saccarola <[email protected]>
Signed-off-by: Christian Brabandt <[email protected]>

Author: saccarosium

runtime/pack/dist/opt/netrw/autoload/netrw/fs.vim

@@ -2,7 +2,7 @@
 " THESE FUNCTIONS DON'T COMMIT TO ANY BACKWARDS COMPATIBILITY. SO CHANGES AND
 " BREAKAGES IF USED OUTSIDE OF NETRW.VIM ARE EXPECTED.
 
-let s:slash = &shellslash ? '/' : '\'
+let s:slash = !exists('+shellslash') || &shellslash ? '/' : '\'
 
 " netrw#fs#PathJoin: Appends a new part to a path taking different systems into consideration {{{