We don't have a sensible use case where we want hex to double, or INF to
double, or any of that. So check the strings for invalid characters and bail
out early. Invalid characters include 'e' and whitespaces too, we don't need
those.

Small chance of things breaking: if the user-exposed calibration matrix
property was specified using hex numbers this will stop working now. I'll take
that risk.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>