pgsql: Don't use abort(3) in libpq's fe-print.c.

Don't use abort(3) in libpq's fe-print.c.

Causing a core dump on out-of-memory seems pretty unfriendly,
and surely is far outside the expected behavior of a general-purpose
library. Just print an error message (as we did already) and return.
These functions unfortunately don't have an error return convention,
but code using them is probably just looking for a quick-n-dirty
print method and wouldn't bother to check anyway.

Although these functions are semi-deprecated, it still seems
appropriate to back-patch this. In passing, also back-patch
b90e6cef1, just to reduce cosmetic differences between the
branches.

Discussion: https://postgr.es/m/3122443.1624735363@sss.pgh.pa.us

Branch
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REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/06a2b2fe55807c120b1e27bc6043244518c7de65

Modified Files
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src/interfaces/libpq/fe-print.c | 85 ++++++++++++++++++++++++-----------------
1 file changed, 49 insertions(+), 36 deletions(-)