patch 8.0.0383: misplaced #ifdef

Problem:    Misplaced #ifdef. (Christ van Willigen)
Solution:   Split assignment.
diff --git a/src/screen.c b/src/screen.c
index 2c9bbef..8514f3e 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -4201,8 +4201,9 @@
 	    /*
 	     * Get a character from the line itself.
 	     */
+	    c = *ptr;
 #ifdef FEAT_LINEBREAK
-	    c0 = c = *ptr;
+	    c0 = *ptr;
 #endif
 #ifdef FEAT_MBYTE
 	    if (has_mbyte)