rand_alnum: fix warning
rand_alnum: fix warning

Change-Id: Ib6782d7828f0c350684c471ed4e71d303b2b9f82
warning: suggest parentheses around ‘&&’ within ‘||’
Reviewed-on: http://gerrit.aldebaran.lan:8080/14946
Reviewed-by: vpaleologue <cb19838a79eb484e12a6960acfbf7c16dfdb80d9@aldebaran-robotics.com>
Reviewed-by: dmerejkowsky <1e4b13000fa8777df4cefd2fd8215187e3deb062@aldebaran-robotics.com>
Tested-by: gerrit

aldebaran/libqi,vbarbaresi/libqi,aldebaran/libqi,aldebaran/libqi,bsautron/libqi