patch 9.0.2126: unused assignments when checking 'listchars'

Problem:  Unused assignments when checking the value of 'listchars'.
Solution: Loop only once when just checking the value.  Add a test to
          check that this change doesn't cause double-free.

closes: #13559

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/screen.c b/src/screen.c
index fd69cf0..ac24a16 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -4726,7 +4726,7 @@
     }
 
     // first round: check for valid value, second round: assign values
-    for (round = 0; round <= 1; ++round)
+    for (round = 0; round <= (apply ? 1 : 0); ++round)
     {
 	if (round > 0)
 	{
@@ -4913,11 +4913,6 @@
 	    wp->w_fill_chars = fill_chars;
 	}
     }
-    else if (is_listchars)
-    {
-	vim_free(lcs_chars.multispace);
-	vim_free(lcs_chars.leadmultispace);
-    }
 
     return NULL;	// no error
 }