xl: Rewrite trim()

xl: Rewrite trim()

This function would produce a NULL output pointer if the input was an
empty string, leading to a crash.

I don't think this is likely to be a security problem, as the two call
sites involve configuration options which callers are unlikely to
expose to other-than-fully-trusted input.

Also, the function would needlessly copy the input string (which I
care about not for performance reasons but because it makes the memory
handling more confusing), and would mishandle strings which contained
only predicate-true characters.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>