patch 8.2.4880: Vim9: misplaced elseif causes invalid memory access

Problem:    Vim9: misplaced elseif causes invalid memory access.
Solution:   Check cs_idx not to be negative.

src/ex_eval.c

@@ -1124,10 +1124,13 @@ ex_else(exarg_T *eap)
 	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 (cstack->cs_idx >= 0)
+    {
+	// 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)

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4880,
 /**/
     4879,
 /**/