patch 9.0.2110: [security]: overflow in ex address parsing

Problem:  [security]: overflow in ex address parsing
Solution: Verify that lnum is positive, before substracting from
          LONG_MAX

[security]: overflow in ex address parsing

When parsing relative ex addresses one may unintentionally cause an
overflow (because LONG_MAX - lnum will overflow for negative addresses).

So verify that lnum is actually positive before doing the overflow
check.

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/testdir/test_excmd.vim

@@ -724,5 +724,9 @@ func Test_write_after_rename()
   bwipe!
 endfunc
 
+" catch address lines overflow
+func Test_ex_address_range_overflow()
+  call assert_fails(':--+foobar', 'E492:')
+endfunc
 
 " vim: shiftwidth=2 sts=2 expandtab