Backend crash using ST_Buffer

[strk]

It looks like someone changed the API of a C function, resulting in a backend crash upon using ST_Buffer old signature:

==8887== Invalid read of size 1 ==8887== at 0x5442B0: pg_detoast_datum (in /usr/lib/postgresql/9.6/bin/postgres) ==8887== by 0x19D7D2A6: buffer (lwgeom_geos.c:809) ==8887== Address 0x0 is not stack'd, malloc'd or (recently) free'd

NOTE: I'm using old signature because upgrade fails (see #4353)

Signature was changed in r17300 so Paul: it is in your plate to fix!