[PATCH] Fix possible underflow in expression (maxoff - 1)

Hi,
The var OffsetNumber maxoff it's like uint16, see at include/storage/off.h
typedef uint16 OffsetNumber;

Within the function _bt_afternewitemoff, at line 641, maxoff is used in an dangerous expression,
without protection.: (maxoff - 1)

The function: PageGetMaxOffsetNumber that initializes maxoff, can return zero.
See at storage/bufpage.h
* PageGetMaxOffsetNumber
* Returns the maximum offset number used by the given page.
* Since offset numbers are 1-based, this is also the number
* of items on the page.
*
* NOTE: if the page is not initialized (pd_lower == 0), we must
* return zero to ensure sane behavior. Accept double evaluation
* of the argument so that we can ensure this.

Surely not the best solution, but it was the best I could think of.

best regards.
Ranier Vilela