patch 8.2.1670: a couple of gcc compiler warnings

Problem:    A couple of gcc compiler warnings.
Solution:   Initialize local variables. (Dominique Pellé, closes #6944)
diff --git a/src/memline.c b/src/memline.c
index 1c594d4..54f8c19 100644
--- a/src/memline.c
+++ b/src/memline.c
@@ -2634,7 +2634,7 @@
     int		n;
     char_u	*props;
     int		new_len = 0;  // init for gcc
-    char_u	*new_line;
+    char_u	*new_line = NULL;
     textprop_T	prop;
 
     // Make two rounds: