patch 8.2.1796: Vim9: invalid memory access with weird function name

Problem:    Vim9: invalid memory access with weird function name. (Dhiraj
            Mishra)
Solution:   Check the name is valid.  Add a test.

src/testdir/test_vim9_func.vim

@@ -1906,6 +1906,32 @@ def Test_closure_in_map()
   delete('XclosureDir', 'rf')
 enddef
 
+def Test_invalid_function_name()
+  var lines =<< trim END
+      vim9script
+      def s: list<string>
+  END
+  CheckScriptFailure(lines, 'E129:')
+
+  lines =<< trim END
+      vim9script
+      def g: list<string>
+  END
+  CheckScriptFailure(lines, 'E129:')
+
+  lines =<< trim END
+      vim9script
+      def <SID>: list<string>
+  END
+  CheckScriptFailure(lines, 'E884:')
+
+  lines =<< trim END
+      vim9script
+      def F list<string>
+  END
+  CheckScriptFailure(lines, 'E488:')
+enddef
+
 def Test_partial_call()
   var Xsetlist = function('setloclist', [0])
   Xsetlist([], ' ', {'title': 'test'})

src/userfunc.c

@@ -2374,6 +2374,7 @@ trans_function_name(
     int		extra = 0;
     lval_T	lv;
     int		vim9script;
+    static char *e_function_name = N_("E129: Function name required");
 
     if (fdp != NULL)
 	CLEAR_POINTER(fdp);
@@ -2401,7 +2402,7 @@ trans_function_name(
     if (end == start)
     {
 	if (!skip)
-	    emsg(_("E129: Function name required"));
+	    emsg(_(e_function_name));
 	goto theend;
     }
     if (end == NULL || (lv.ll_tv != NULL && (lead > 2 || lv.ll_range)))
@@ -2517,6 +2518,12 @@ trans_function_name(
 	}
 	len = (int)(end - lv.ll_name);
     }
+    if (len <= 0)
+    {
+	if (!skip)
+	    emsg(_(e_function_name));
+	goto theend;
+    }
 
     // In Vim9 script a user function is script-local by default.
     vim9script = ASCII_ISUPPER(*start) && in_vim9script();

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1796,
 /**/
     1795,
 /**/