Cast to an unsigned type before negating

Cast to an unsigned type before negating

llvm's ubsan reported:
runtime error: negation of -9223372036854775808 cannot be represented in type
'long'; cast to an unsigned type to negate this value to itself

Found using afl

Reviewed-by: Rich Salz <rsalz@openssl.org>
GH: #1325
(cherry picked from commit 1618679ac478c8f41fc5f320fb4d8a33883b3868)