Fix BLI_str_utf8_as_unicode_step reading past intended bounds

Add a string length argument to BLI_str_utf8_as_unicode_step to prevent
reading past the buffer bounds or the intended range since some callers
of this function take a string length to operate on part of the string.

Font drawing for example didn't respect the length argument,
potentially causing a buffer over-read with multi-byte characters
that could read past the end of the string.

The following command would read 5 bytes past the end of the input.

`BLF_draw(font_id, (char[]){252}, 1);`

In practice strings are typically null terminated so this didn't crash
reading past buffer bounds.

Nevertheless, this wasn't correct and could cause bugs in the future.

Clamping by the length now has the same behavior as a null byte.

Add test to ensure this is working as intended.

1 files changed, 4 insertions, 2 deletions

diff --git a/source/blender/blenlib/BLI_string_utf8.h b/source/blender/blenlib/BLI_string_utf8.h
index e1d7e2c58f7..b936e39731d 100644
--- a/source/blender/blenlib/BLI_string_utf8.h
+++ b/source/blender/blenlib/BLI_string_utf8.h
@@ -43,8 +43,10 @@ unsigned int BLI_str_utf8_as_unicode_and_size(const char *__restrict p, size_t *
     ATTR_NONNULL();
 unsigned int BLI_str_utf8_as_unicode_and_size_safe(const char *__restrict p,
                                                    size_t *__restrict index) ATTR_NONNULL();
-unsigned int BLI_str_utf8_as_unicode_step(const char *__restrict p, size_t *__restrict index)
-    ATTR_NONNULL();
+unsigned int BLI_str_utf8_as_unicode_step(const char *__restrict p,
+                                          size_t p_len,
+                                          size_t *__restrict index) ATTR_NONNULL(1, 3);
+
 size_t BLI_str_utf8_from_unicode(unsigned int c, char *outbuf);
 size_t BLI_str_utf8_as_utf32(char32_t *__restrict dst_w,
                              const char *__restrict src_c,