patch 8.1.0751: some regexp errors are not tested

Problem:    Some regexp errors are not tested.
Solution:   Add a test function.
diff --git a/src/testdir/test_regexp_latin.vim b/src/testdir/test_regexp_latin.vim
index a8fa64d..1524945 100644
--- a/src/testdir/test_regexp_latin.vim
+++ b/src/testdir/test_regexp_latin.vim
@@ -72,3 +72,15 @@
   call assert_fails('call search("\\%#=2\\(e\\1\\)")', 'E65:')
   bwipe!
 endfunc
+
+func Test_multi_failure()
+  set re=1
+  call assert_fails('/a**', 'E61:')
+  call assert_fails('/a*\+', 'E62:')
+  call assert_fails('/a\{a}', 'E554:')
+  set re=2
+  call assert_fails('/a**', 'E871:')
+  call assert_fails('/a*\+', 'E871:')
+  call assert_fails('/a\{a}', 'E870:')
+  set re=0
+endfunc
diff --git a/src/version.c b/src/version.c
index b0f4886..f84da1d 100644
--- a/src/version.c
+++ b/src/version.c
@@ -796,6 +796,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    751,
+/**/
     750,
 /**/
     749,