Fix incorrect return value in brin_minmax_multi_distance_numeric().

The result of "DirectFunctionCall1(numeric_float8, d)" is already in
Datum form, but the code was incorrectly applying PG_RETURN_FLOAT8()
to it.  On machines where float8 is pass-by-reference, this would
result in complete garbage, since an unpredictable pointer value
would be treated as an integer and then converted to float.  It's not
entirely clear how much of a problem would ensue on 64-bit hardware,
but certainly interpreting a float8 bitpattern as uint64 and then
converting that to float isn't the intended behavior.

As luck would have it, even the complete-garbage case doesn't break
BRIN indexes, since the results are only used to make choices about
how to merge values into ranges: at worst, we'd make poor choices
resulting in an inefficient index.  Doubtless that explains the lack
of field complaints.  However, users with BRIN indexes that use the
numeric_minmax_multi_ops opclass may wish to reindex in hopes of
making their indexes more efficient.

Author: Peter Eisentraut <peter@eisentraut.org>
Co-authored-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/2093712.1753983215@sss.pgh.pa.us
Backpatch-through: 14

src/backend/access/brin/brin_minmax_multi.c

@@ -2032,7 +2032,7 @@ brin_minmax_multi_distance_numeric(PG_FUNCTION_ARGS)
 
 	d = DirectFunctionCall2(numeric_sub, a2, a1);	/* a2 - a1 */
 
-	PG_RETURN_FLOAT8(DirectFunctionCall1(numeric_float8, d));
+	PG_RETURN_DATUM(DirectFunctionCall1(numeric_float8, d));
 }
 
 /*