fix(mbyte): fix bugs in utf_cp_*_off() functions

Problems:
- Illegal bytes after valid UTF-8 char cause utf_cp_*_off() to fail.
- When stream isn't NUL-terminated, utf_cp_*_off() may go over the end.
Solution: Don't go over end of the char of end of the string.

1 files changed, 5 insertions, 0 deletions

diff --git a/src/nvim/mbyte_defs.h b/src/nvim/mbyte_defs.h
index 058ec353e5..8670a0595d 100644
--- a/src/nvim/mbyte_defs.h
+++ b/src/nvim/mbyte_defs.h
@@ -66,3 +66,8 @@ typedef struct {
   char *ptr;     ///< Pointer to the first byte of the character.
   CharInfo chr;  ///< Information about the character.
 } StrCharInfo;
+
+typedef struct {
+  int8_t begin_off;  ///< Offset to the first byte of the codepoint.
+  int8_t end_off;    ///< Offset to one past the end byte of the codepoint.
+} CharBoundsOff;