Use INT32_MIN to check for the smallest possible 32-bit signed value.

Use INT32_MIN to check for the smallest possible 32-bit signed value.

-2147483648 provokes warnings, for various reasons involving the
definition of a constant in C, the rules of picking the type of a
constant, and the rules for determining the type of an expression
involving an integral value and the - operator.  (Note that
-2147483648isn't a decimal-constant in C90 or C99, as a decimal-constant
can't contain a -.  Therefore, it's a constant expression, consisting of
the - operator applied to the constant 2147483648.)

So we use INT32_MIN, which should at least make an effort to avoid those
warnings.