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.
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