Fix: path: handle truncation in utils_partial_realpath()

Fix: path: handle truncation in utils_partial_realpath()

gcc warns that:
  warning: 'char* strncpy(char*, const char*, size_t)' specified bound 4096 equals destination size [-Wstringop-truncation]

Return an error when lttng_strncpy reports that a truncation occurred.

Change-Id: I03889d6d275566413df2848974e4d3ad83565b17
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>