Window manager crashes for wrong pixel value

If the argument for 'f' is not an integer the whole window manager crashes. 
This does not seem correct.
I think a message to stderr via fprintf would be a better solution.

Example:
```
xsetroot -name "^fff^hello"
```
Another Example:
```
xsetroot -name "^c#FFfffffff^foobar^d^barfoo"
```