Do not overflow when deleting Unicode in GuiTextBox()

[SirNate0]

#399

I have not tested it (I don't use Unicode input myself), I just saw the issue and thought it should be a simple fix.

I also noted that there are some inconsistencies in whether autoCursorCooldownCounter is tested > or >= RAYGUI_TEXTBOX_AUTO_CURSOR_COOLDOWN. Were these deliberate, or should they be changed as well? And was it intended that the re-triggering on hold would not happen until 2*RAYGUI_TEXTBOX_AUTO_CURSOR_COOLDOWN (because of the autoCursorDelayCounter++ at the first line of the if, the %RAYGUI_TEXTBOX_AUTO_CURSOR_COOLDOWN will only be zero when it is 2* and not 1*)?