Add overflow checks to int4 and int8 versions of generate_series().

The previous code went into an infinite loop after overflow.  In fact,
an overflow is not really an error; it just means that the current
value is the last one we need to return.  So, just arrange to stop
immediately when overflow is detected.

Back-patch all the way.

src/backend/utils/adt/int.c

@@ -1470,6 +1470,10 @@ generate_series_step_int4(PG_FUNCTION_ARGS)
 		/* increment current in preparation for next iteration */
 		fctx->current += fctx->step;
 
+		/* if next-value computation overflows, this is the final result */
+		if (SAMESIGN(result, fctx->step) && !SAMESIGN(result, fctx->current))
+			fctx->step = 0;
+
 		/* do when there is more left to send */
 		SRF_RETURN_NEXT(funcctx, Int32GetDatum(result));
 	}

src/backend/utils/adt/int8.c

@@ -1268,6 +1268,10 @@ generate_series_step_int8(PG_FUNCTION_ARGS)
 		/* increment current in preparation for next iteration */
 		fctx->current += fctx->step;
 
+		/* if next-value computation overflows, this is the final result */
+		if (SAMESIGN(result, fctx->step) && !SAMESIGN(result, fctx->current))
+			fctx->step = 0;
+
 		/* do when there is more left to send */
 		SRF_RETURN_NEXT(funcctx, Int64GetDatum(result));
 	}