BUILD/MINOR: cli: shut a minor gcc warning in "show fd"

BUILD/MINOR: cli: shut a minor gcc warning in "show fd"

Recent commit 7a4a0ac ("MINOR: cli: add a new "show fd" command") introduced
a warning when building at -O2 and above. The compiler doesn't know if a
variable's value might have changed between two if blocks so warns that some
values might be used uninitialized, which is not the case. Let's simply
initialize them to shut the warning.