Fix out-of-bound access in KeySymToUcs4()

Array keysym_to_unicode_590_5fe is only valid for range [0x590, 0x5fe] but current lower-bound is checked against 0x589.

So invalid values from 0x58a to 0x58f are being allowed by current check.

If any of these invalid value is passed as keysym, keysym - 0x590 would underflow.