package/pure-ftpd: fix CVE-2020-9365

An issue was discovered in Pure-FTPd 1.0.49. An out-of-bounds (OOB) read
has been detected in the pure_strcmp function in utils.c.

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>