shared: path-util - try to make PATH_FORECH_PREFIX look less wrong

shared: path-util - try to make PATH_FORECH_PREFIX look less wrong

We replace the idiom "X && !(*foo = 0)" with "X && ((*foo = 0), true)".

This is not a functional change, but should hopefully make it less
likely that people and static analyzers believe there is a typo here
(i.e., to make it clear that the intention was not "X && *foo != 0").

Thanks to David Herrmann for the suggestion.