patch 8.1.0959: sorting large numbers is not tested

Problem:    Sorting large numbers is not tested and does not work properly.
Solution:   Add test.  Fix comparing lines with and without a number.
            (Dominique Pelle, closes #4017)

src/testdir/test_sort.vim

@@ -1222,6 +1222,77 @@ func Test_sort_cmd()
   enew!
 endfunc
 
+func Test_sort_large_num()
+  new
+  a
+-2147483648
+-2147483647
+
+-1
+0
+1
+-2147483646
+2147483646
+2147483647
+2147483647
+-2147483648
+abc
+
+.
+  " Numerical sort. Non-numeric lines are ordered before numerical lines.
+  " Ordering of non-numerical is stable.
+  sort n
+  call assert_equal(['',
+  \                  'abc',
+  \                  '',
+  \                  '-2147483648',
+  \                  '-2147483648',
+  \                  '-2147483647',
+  \                  '-2147483646',
+  \                  '-1',
+  \                  '0',
+  \                  '1',
+  \                  '2147483646',
+  \                  '2147483647',
+  \                  '2147483647'], getline(1, '$'))
+  bwipe!
+
+  if has('num64')
+    new
+    a
+-9223372036854775808
+-9223372036854775807
+
+-1
+0
+1
+-9223372036854775806
+9223372036854775806
+9223372036854775807
+9223372036854775807
+-9223372036854775808
+abc
+
+.
+    sort n
+    call assert_equal(['',
+    \                  'abc',
+    \                  '',
+    \                  '-9223372036854775808',
+    \                  '-9223372036854775808',
+    \                  '-9223372036854775807',
+    \                  '-9223372036854775806',
+    \                  '-1',
+    \                  '0',
+    \                  '1',
+    \                  '9223372036854775806',
+    \                  '9223372036854775807',
+    \                  '9223372036854775807'], getline(1, '$'))
+    bwipe!
+  endif
+endfunc
+
+
 func Test_sort_cmd_report()
     enew!
     call append(0, repeat([1], 3) + repeat([2], 3) + repeat([3], 3))