patch 8.1.0934: invalid memory access in search pattern

Problem:    Invalid memory access in search pattern. (Kuang-che Wu)
Solution:   Check for incomplete equivalence class. (closes #3970)
diff --git a/src/testdir/test_regexp_latin.vim b/src/testdir/test_regexp_latin.vim
index bcac6c7..09104a1 100644
--- a/src/testdir/test_regexp_latin.vim
+++ b/src/testdir/test_regexp_latin.vim
@@ -97,3 +97,10 @@
   " This will be slow...
   call assert_fails('call search("\\v((n||<)+);")', 'E363:')
 endfunc
+
+func Test_get_equi_class()
+  new
+  " Incomplete equivalence class caused invalid memory access
+  s/^/[[=
+  call assert_equal(1, search(getline(1)))
+endfunc