patch 8.2.5017: gcc 12.1 warns for uninitialized variable

Problem:    Gcc 12.1 warns for uninitialized variable.
Solution:   Initialize the variable. (closes #10476)
diff --git a/src/evalvars.c b/src/evalvars.c
index e83c50d..f7939cb 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -2023,7 +2023,7 @@
 {
     hashtab_T	*ht;
     hashitem_T	*hi;
-    char_u	*varname;
+    char_u	*varname = NULL;  // init to shut up gcc
     dict_T	*d;
     dictitem_T	*di;