popen() requires check for fdopen() failure

Move fdopen() up near other resource allocation like malloc(); do proper
deallocation on failure later on in the function.

Submitted by: Ramachandra Topannavar <rtopannavar@panasas.com>
Reviewed by: jilles
Approved by: jhb (mentor)
MFC after: 2 weeks
Sponsored by: Panasas, Inc.
Differential Revision: https://reviews.freebsd.org/D4126

M lib/libc/gen/popen.c