Avoid signed integer overflow due to unary negation

The current X509_print_ex() tries too hard pretty printing negative
serialNumbers (which shouldn't occur in the first place). In particular,
negating LONG_MAX leads to signed overflow. Ditch the code dealing with
negative serialNumbers representable as long and fall back to the long
form printing. This simplifies the code and fixes

oss-fuzz #49944

with/ok jsing

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