patch 8.1.0935: old regexp engine may use invalid buffer

Problem:    Old regexp engine may use invalid buffer for 'iskeyword' or
            uninitialized buffer pointer. (Kuang-che Wu)
Solution:   Set rex.reg_buf when compiling the pattern. (closes #3972)

src/testdir/test_regexp_latin.vim

@@ -104,3 +104,19 @@ func Test_get_equi_class()
   s/^/[[=
   call assert_equal(1, search(getline(1)))
 endfunc
+
+func Test_rex_init()
+  set noincsearch
+  set re=1
+  new
+  setlocal iskeyword=a-z
+  call setline(1, ['abc', 'ABC'])
+  call assert_equal(1, search('[[:keyword:]]'))
+  new
+  setlocal iskeyword=A-Z
+  call setline(1, ['abc', 'ABC'])
+  call assert_equal(2, search('[[:keyword:]]'))
+  bwipe!
+  bwipe!
+  set re=0
+endfunc