Suppress integer-overflow compiler warning for inconsistent sun_len.
On AIX 7.1, struct sockaddr_un is declared to be 1025 bytes long,
but the sun_len field that should hold the length is only a byte.
Clamp the value we try to store to ensure it will fit in the field.
(This coding might need adjustment if there are any machines out
there where sun_len is as wide as size_t; but a preliminary survey
suggests there's not, so let's keep it simple.)
Discussion: https://postgr.es/m/
2781112.
1644819528@sss.pgh.pa.us