vim-patch:9.2.1041: vim_strbyte() can be improved (#41784)

Problem:  vim_strbyte() can be improved
Solution: Replace the byte-by-byte scan in vim_strbyte() with strchr(),
          preserving its byte-range and NUL behavior (Yasuhiro Matsumoto).

closes: vim/vim#21240

df4ef959d5

Co-authored-by: Yasuhiro Matsumoto <mattn.jp@gmail.com>

test/old/testdir/test_regexp_utf8.vim

@@ -1,6 +1,33 @@
 " Tests for regexp in utf8 encoding
 
-source shared.vim
+func Test_regexp_literal_byte_search()
+  let save_encoding = &encoding
+  let save_re = &regexpengine
+  try
+    "for enc in ['latin1', 'utf-8']
+    for enc in ['utf-8']
+      let &encoding = enc
+      for engine in [1, 2]
+        let &regexpengine = engine
+        let chars = ['z', nr2char(0x7f), nr2char(0xff)]
+        if enc == 'utf-8'
+          call add(chars, nr2char(0x100))
+        endif
+        for c in chars
+          let prefix = repeat('x', 4096)
+          call assert_equal(-1, match('', c))
+          call assert_equal(-1, match(prefix, c))
+          call assert_equal(0, match(c .. prefix, c))
+          call assert_equal(4096, match(prefix .. c, c))
+        endfor
+        call assert_equal(-1, match('xxx', '\%d0'))
+      endfor
+    endfor
+  finally
+    let &encoding = save_encoding
+    let &regexpengine = save_re
+  endtry
+endfunc
 
 func s:equivalence_test()
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