patch 9.0.1869: Coverity warns about unitialized var

Problem:  Coverity warns about unitialized var
Solution: initialize it

closes: #13029

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Ernie Rael <errael@raelity.com>
diff --git a/src/eval.c b/src/eval.c
index 4e1dbde..ec8d3ba 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1025,7 +1025,7 @@
     int		len;
     hashtab_T	*ht = NULL;
     int		quiet = flags & GLV_QUIET;
-    int		writing;
+    int		writing = 0;
     int		vim9script = in_vim9script();
 
     // Clear everything in "lp".