libcutils: ashmem print error message for invalid fd

NB: We decided to not sniff for the constant saved __ashmem_rdev in
the early error path; requiring either the use of atomic operations,
or acquiring a lock to do it correctly. The heroics are not worth it.

Bug: 26871259
Change-Id: I46249838850ae32063eb5b7d08c731c5bb0fbf6b