memif: fix vector overflow when copying strings

memif: fix vector overflow when copying strings

When memif sends back socket messages containing strings, we copy
vectors into C-string. Unfortunately, most vectors are not
null-terminated, causing strncpy() read overflow. Moreover, strncpy()
does not null-terminate string in case of max length reached.
This patch introduces helpers to safely copy strings from vectors.

Type: fix
Fixes: d6042d4f1ea0baf02bc87c72960a331a9e08dfab

Change-Id: I38489ec8d2a5d4a42b9abde1aa3dfdbd06ebe024
Signed-off-by: Benoît Ganne <bganne@cisco.com>