Fix thinko introduced in 6b423ec67

Fix thinko introduced in 6b423ec67

As pointed out by Dean Rasheed, we really should be using tmp >
-(PG_INTNN_MIN / 10) rather than tmp > (PG_INTNN_MAX / 10) for checking
for overflows in the accumulation in the pg_strtointNN functions.  This
does happen to be the same number when dividing by 10, but there is a
pending patch which adds other bases and this is not the same number if we
were to divide by 2 rather than 10, for example.  If the base 2 parsing
was to follow this example then we could accidentally think a string
containing the value of PG_INT32_MIN was an overflow in pg_strtoint32.
Clearly that shouldn't overflow.

This does not fix any actual live bugs, only some bad examples of overflow
checks for future bases.

Reported-by: Dean Rasheed
Discussion: https://postgr.es/m/CAEZATCVEtwfhdm-K-etZYFB0=qsR0nT6qXta_W+GQx4RYph1dg@mail.gmail.com