patch 8.2.2028: Coverity warns for using an uninitialized variable

Problem:    Coverity warns for using an uninitialized variable.
Solution:   Initialize to NULL.
diff --git a/src/eval.c b/src/eval.c
index 0365adc..33976d2 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -791,7 +791,7 @@
     listitem_T	*ni;
     char_u	*key = NULL;
     int		len;
-    hashtab_T	*ht;
+    hashtab_T	*ht = NULL;
     int		quiet = flags & GLV_QUIET;
 
     // Clear everything in "lp".