SDL_joystick.c: cast the arguments to nibble() to unsigned char.

src/joystick/SDL_joystick.c

@@ -2646,18 +2646,18 @@ void SDL_JoystickGetGUIDString(SDL_JoystickGUID guid, char *pszGUID, int cbGUID)
  * Input  : c -
  * Output : unsigned char
  *-----------------------------------------------------------------------------*/
-static unsigned char nibble(char c)
+static unsigned char nibble(unsigned char c)
 {
     if ((c >= '0') && (c <= '9')) {
-        return (unsigned char)(c - '0');
+        return (c - '0');
     }
 
     if ((c >= 'A') && (c <= 'F')) {
-        return (unsigned char)(c - 'A' + 0x0a);
+        return (c - 'A' + 0x0a);
     }
 
     if ((c >= 'a') && (c <= 'f')) {
-        return (unsigned char)(c - 'a' + 0x0a);
+        return (c - 'a' + 0x0a);
     }
 
     /* received an invalid character, and no real way to return an error */
@@ -2681,7 +2681,7 @@ SDL_JoystickGUID SDL_JoystickGetGUIDFromString(const char *pchGUID)
 
     p = (Uint8 *)&guid;
     for (i = 0; (i < len) && ((p - (Uint8 *)&guid) < maxoutputbytes); i+=2, p++) {
-        *p = (nibble(pchGUID[i]) << 4) | nibble(pchGUID[i+1]);
+        *p = (nibble((unsigned char)pchGUID[i]) << 4) | nibble((unsigned char)pchGUID[i+1]);
     }
 
     return guid;