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]

Error-out when snprintf indicates that a truncation occurred.

Change-Id: I8c514da6b0ccb1a59d1555b02bfea2dd3c57febb
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>