patch 8.2.3425: warning for using uninitialized variable

Problem:    Warning for using uninitialized variable.
Solution:   Initialize it. (John Marriott)
diff --git a/src/screen.c b/src/screen.c
index 6a9f632..6855bba 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -4787,8 +4787,8 @@
     int		round, i, len, entries;
     char_u	*p, *s;
     int		c1 = 0, c2 = 0, c3 = 0;
-    char_u	*last_multispace;	// Last occurrence of "multispace:"
-    int		multispace_len = 0;	// Length of lcs-multispace string
+    char_u	*last_multispace = NULL; // Last occurrence of "multispace:"
+    int		multispace_len = 0;	 // Length of lcs-multispace string
     struct charstab
     {
 	int	*cp;