bindings: when implanting a string, make sure to use positive values

This fixes https://savannah.gnu.org/bugs/?54712.

src/winio.c

@@ -291,7 +291,7 @@ void unget_kbinput(int kbinput, bool metakey)
 void implant(const char *string)
 {
 	for (int i = strlen(string); i > 0; i--)
-		put_back(string[i - 1]);
+		put_back((unsigned char)string[i - 1]);
 }
 #endif