gdbserver: Fix vAttach response when attaching is not supported

gdbserver: Fix vAttach response when attaching is not supported

handle_v_attach calls attach_inferior, which says:

  "return -1 if attaching is unsupported, 0 if it succeeded, and call
  error() otherwise."

So if attach_inferior return != 0, we have the unsupported case,
meaning we should return the empty packet instead of an error.

In practice, this shouldn't trigger, as vAttach support is supposed to
be reported via qSupported.  But it doesn't hurt to be pedantic here.

Change-Id: I99cce6fa678f2370571e6bca0657451300956127
Approved-By: Tom Tromey <tom@tromey.com>