Fix possible bogus array out of bonds (src/backend/access/brin/brin_minmax_multi.c)

Hi,

At function has_matching_range, if variable ranges->nranges == 0,
we exit quickly with a result equal to false.

This means that nranges can be zero.
It occurs then that it is possible then to occur an array out of bonds, in
the initialization of the variable maxvalue.
So if nranges is equal to zero, there is no need to initialize minvalue and
maxvalue.

The patch tries to fix it, avoiding possible errors by using maxvalue.

regards,
Ranier Vilela