patch 8.2.2121: internal error when using \ze before \zs in a pattern

Problem:    Internal error when using \ze before \zs in a pattern.
Solution:   Check the end is never before the start. (closes #7442)
diff --git a/src/testdir/test_regexp_latin.vim b/src/testdir/test_regexp_latin.vim
index d1885ab..0b00244 100644
--- a/src/testdir/test_regexp_latin.vim
+++ b/src/testdir/test_regexp_latin.vim
@@ -911,6 +911,13 @@
   bwipe!
 endfunc
 
+func Test_ze_before_zs()
+  call assert_equal('', matchstr(' ', '\%#=1\ze \zs'))
+  call assert_equal('', matchstr(' ', '\%#=2\ze \zs'))
+  call assert_equal(repeat([''], 10), matchlist(' ', '\%#=1\ze \zs'))
+  call assert_equal(repeat([''], 10), matchlist(' ', '\%#=2\ze \zs'))
+endfunc
+
 " Check for detecting error
 func Test_regexp_error()
   call assert_fails("call matchlist('x x', '\\%#=1 \\zs*')", 'E888:')