Prevent length wraparound in RichString_rewind()

The computed length could wrap to a negative integer, causing undefined
behavior.

(Currently it would cause out-of-bound array access due to a signed
length property in RichString. The length property should be migrated
to an unsigned size_t type, but that would be done in a later commit.)

Signed-off-by: Kang-Che Sung <[email protected]>

RichString.c

@@ -65,7 +65,7 @@ static void RichString_setLen(RichString* this, int len) {
 }
 
 void RichString_rewind(RichString* this, int count) {
-   RichString_setLen(this, this->chlen - count);
+   RichString_setLen(this, this->chlen > count ? this->chlen - count : 0);
 }
 
 #ifdef HAVE_LIBNCURSESW