check_apt: fix memset

Fix memset introduced by commit 9ce7369 ("plugins/check_apt.c - Print uninitialized ereg").

Signed-off-by: Sebastian Herbszt <herbszt@gmx.de>
check_ntp: fix null termination

Fix null termination introduced by commit a04df3e ("plugins/check_ntp.c - Verify struct from response").

Signed-off-by: Sebastian Herbszt <herbszt@gmx.de>
check_real: fix null termination

Fix null termination introduced by commit b61f51a ("plugins/check_real.c - recv string null terminate").

Signed-off-by: Sebastian Herbszt <herbszt@gmx.de>