Fix character type detection in commonmark.c

- Implement cmark_isalpha.
- Check for ASCII character before implicit cast to char.
- Use internal ctype functions in commonmark.c.

Fixes test failures on Windows and undefined behavior.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/cmark_ctype.h b/src/cmark_ctype.h
index f803946..9a07618 100644
--- a/src/cmark_ctype.h
+++ b/src/cmark_ctype.h
@@ -17,6 +17,8 @@ int cmark_isalnum(char c);
 
 int cmark_isdigit(char c);
 
+int cmark_isalpha(char c);
+
 #ifdef __cplusplus
 }
 #endif