Fix sscanf limits in pg_dump

Fix sscanf limits in pg_dump

Make sure that the string parsing is limited by the size of the
destination buffer.

The buffer is bounded by MAXPGPATH, and thus the limit must be
inserted via preprocessor expansion and the buffer increased by
one to account for the terminator. There is no risk of overflow
here, since in this case, the buffer scanned is smaller than the
destination buffer.

Backpatch all the way down to 9.6.

Reviewed-by: Tom Lane
Discussion: https://postgr.es/m/B14D3D7B-F98C-4E20-9459-C122C67647FB@yesql.se
Backpatch-through: 9.6