pgsql: SPI_cursor_open failed to enforce that only read-only queries

Log Message:
-----------
SPI_cursor_open failed to enforce that only read-only queries could be
executed in read_only mode. This could lead to various relatively-subtle
failures, such as an allegedly stable function returning non-stable results.
Bug goes all the way back to the introduction of read-only mode in 8.0.
Per report from Gaetano Mendola.

Modified Files:
--------------
pgsql/src/backend/executor:
spi.c (r1.172 -> r1.173)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/executor/spi.c.diff?r1=1.172&r2=1.173)