sd-resolve: do not assert on packet size received over a socket

sd-resolve: do not assert on packet size received over a socket

This is external data, even if trusted. We should not assert on it, but verify
and return proper error instead, which assert_return does. In particular,
write(2) says that a partial write could occur when interupted by a signal.
When compiled with asserts disabled, we could access memory outside of the
allocated buffer.

CID #1237671.
Follow-up for 1a96c8e1ccb06f87b6bfaff4639390ecd00af588.