RichString: avoid signed integer misuse

RichString.c

@@ -86,7 +86,7 @@ static inline void RichString_writeFrom(RichString* this, int attrs, const char*
    int newLen = from + len;
    RichString_setLen(this, newLen);
    for (int i = from, j = 0; i < newLen; i++, j++) {
-      this->chptr[i] = (data_c[j] >= 32 ? data_c[j] : '?') | attrs;
+      this->chptr[i] = (((unsigned char)data_c[j]) >= 32 ? ((unsigned char)data_c[j]) : '?') | attrs;
    }
    this->chptr[newLen] = 0;
 }