Specify RGB Divisor Explicitly in the color Key

The documentation of xcolor explains that this key is not about red,
green and blue in a scale of 255, but in a scale defined either by the
sum of all the values or an explicit scale indicated by the syntax. If
the sum of all the values is 0, like in the bug report, then it causes
an arithmetic overflow; yet, besides this bug, the wrong interpretation
causes another issue when setting an object color to white (which is
probably why this was not spotted before): the color will be interpreted
as (0.33,0.33,0.33) in RGB.

BUG: 340213
FIXED-IN: 4.14.3