openamp: fixing buffer overflow in rpmsg_init_ept function

GCC detects that in the rpmsg_init_ept function ept->name
string NULL character may be overriden by strncpy.
This could cause a buffer overflow (stringop-truncation warning).

This commit fixes this issue.

Change-Id: I74a890589e63d5c192cd8ac597f51b3d80a2dc39
Signed-off-by: Lakshmi Kailasanathan <lakshmi.kailasanathan@arm.com>
Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>