patch 8.2.2977: crash when using a null function reference

Problem:    Crash when using a null function reference. (Naohiro Ono)
Solution:   Check for an invalid function name. (closes #8367)
diff --git a/src/testdir/test_functions.vim b/src/testdir/test_functions.vim
index 1309554..d63bec4 100644
--- a/src/testdir/test_functions.vim
+++ b/src/testdir/test_functions.vim
@@ -2174,9 +2174,11 @@
   call assert_fails("call call('Mylen', [], 0)", 'E715:')
   call assert_fails('call foo', 'E107:')
 
-  " This once caused a crash.
+  " These once caused a crash.
   call call(test_null_function(), [])
   call call(test_null_partial(), [])
+  call assert_fails('call test_null_function()()', 'E1192:')
+  call assert_fails('call test_null_partial()()', 'E117:')
 endfunc
 
 func Test_char2nr()