Fix error handling in gradient parsing

For a malformed gradient specification, the parsing code would return
-1. However, since the parsing function returns bool, not int, that
value would be cast to true: the opposite of the intended. Fix by
returning false instead.

Pick-to: 6.12
Change-Id: I04d56b82f254bf8e290ab465bfc60a9db2dd2660
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>