cygwin: Remove comparison of 'this' to 'NULL' in _pinfo::cwd

Fix all callers.

winsup/cygwin/fhandler_process.cc

@@ -498,7 +498,7 @@ format_process_cwd (void *data, char *&destbuf)
       cfree (destbuf);
       destbuf = NULL;
     }
-  destbuf = p->cwd (fs);
+  destbuf = p ? p->cwd (fs) : NULL;
   if (!destbuf || !*destbuf)
     {
       destbuf = cstrdup ("<defunct>");

winsup/cygwin/pinfo.cc

@@ -930,7 +930,7 @@ char *
 _pinfo::cwd (size_t& n)
 {
   char *s = NULL;
-  if (!this || !pid)
+  if (!pid)
     return NULL;
   if (ISSTATE (this, PID_NOTCYGWIN))
     {