patch 8.2.4873: Vim9: using "else" differs from using "endif/if !cond"

Problem:    Vim9: using "else" differs from using "endif/if !cond".
Solution:   Leave the block and enter another one. (closes #10320)
diff --git a/src/ex_eval.c b/src/ex_eval.c
index e9dbd10..7b4ac17 100644
--- a/src/ex_eval.c
+++ b/src/ex_eval.c
@@ -1124,6 +1124,11 @@
 	skip = TRUE;
     }
 
+    // Variables declared in the previous block can no longer be
+    // used.  Needs to be done before setting "cs_flags".
+    leave_block(cstack);
+    enter_block(cstack);
+
     // if skipping or the ":if" was TRUE, reset ACTIVE, otherwise set it
     if (skip || cstack->cs_flags[cstack->cs_idx] & CSF_TRUE)
     {