In psql, avoid leaking a PGresult after a query is cancelled.

After a query cancel, the tail end of ExecQueryAndProcessResults
took care to clear any not-yet-read PGresults; but it forgot about
the one it has already read.  There would only be such a result
when handling a multi-command string made with "\;", so that you'd
have to cancel an earlier command in such a string to reach the
bug at all.  Even then, there would only be leakage of a single
PGresult per cancel, so it's not surprising nobody noticed this.
But a leak is a leak.

Noted while re-reviewing 90f517821, but this is independent of that:
it dates to 7844c9918.  Back-patch to v15 where that came in.

src/bin/psql/common.c

@@ -1812,6 +1812,8 @@ ExecQueryAndProcessResults(const char *query,
 
 		if (cancel_pressed)
 		{
+			/* drop this next result, as well as any others not yet read */
+			ClearOrSaveResult(result);
 			ClearOrSaveAllResults();
 			break;
 		}