util/readline: Add braces to fix checkpatch errors

Add braces to fix errors issued by checkpatch.pl tool
"ERROR: braces {} are necessary for all arms of this statement"
Within "util/readline.c" file
Message-Id: <20190330112142.14082-1-jbi.octave@gmail.com>

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>