package/libvncserver: fix CVE-2019-15681

LibVNC commit before d01e1bb4246323ba6fcee3b82ef1faa9b1dac82a contains a
memory leak (CWE-655) in VNC server code, which allow an attacker to
read stack memory and can be abused for information disclosure. Combined
with another vulnerability, it can be used to leak stack memory and
bypass ASLR. This attack appear to be exploitable via network
connectivity. These vulnerabilities have been fixed in commit
d01e1bb4246323ba6fcee3b82ef1faa9b1dac82a

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>