Use SPITupleTable->numvals when available (#1037)

Starting from PostgreSQL, SPI tuple table contains number of rows
returned in `numvals`, with `SPI_processed` still available for
"historical reasons":

"The fields tupdesc, vals, and numvals can be used by SPI callers; the
remaining fields are internal. vals is an array of pointers to rows. The
number of rows is given by numvals (for somewhat historical reasons,
this count is also returned in SPI_processed)."

(https://www.postgresql.org/docs/current/spi-spi-execute.html)

Also:

"Some utility commands (COPY, CREATE TABLE AS) don't return a row set,
so SPI_tuptable is NULL, but they still return the number of rows
processed in SPI_processed."

So we're following this logic.

This is to ensure we're not relying on "historical" API going forward.

pgx/src/spi.rs

@@ -578,7 +578,17 @@ impl<'a> SpiClient<'a> {
                     Some(pg_sys::SPI_tuptable)
                 }
             },
+            #[cfg(any(feature = "pg11", feature = "pg12"))]
             size: unsafe { pg_sys::SPI_processed as usize },
+            #[cfg(not(any(feature = "pg11", feature = "pg12")))]
+            // SAFETY: no concurrent access
+            size: unsafe {
+                if pg_sys::SPI_tuptable.is_null() {
+                    pg_sys::SPI_processed as usize
+                } else {
+                    (*pg_sys::SPI_tuptable).numvals as usize
+                }
+            },
             current: -1,
         })
     }