patch 8.1.0326: screen dump does not consider NUL and space equal

Problem:    Screen dump does not consider NUL and space equal.
Solution:   Use temp variables instead of character from cell.
diff --git a/src/terminal.c b/src/terminal.c
index 66a7369..8524560 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -3939,9 +3939,9 @@
 		    c = (c == NUL) ? ' ' : c;
 		    pc = (pc == NUL) ? ' ' : pc;
 		}
-		if (cell.chars[i] != prev_cell.chars[i])
+		if (c != pc)
 		    same_chars = FALSE;
-		if (cell.chars[i] == NUL || prev_cell.chars[i] == NUL)
+		if (c == NUL || pc == NUL)
 		    break;
 	    }
 	    same_attr = vtermAttr2hl(cell.attrs)