Fix a bug in ecp_nistp224.c.
felem_neg does not produce an output within the tight bounds suitable for felem_contract. This affects build configurations which set enable-ec_nistp_64_gcc_128. point_double and point_add, in the non-z*_is_zero cases, tolerate and fix up the wider bounds, so this only affects point_add calls where the other point is infinity. Thus it only affects the final addition in arbitrary-point multiplication, giving the wrong y-coordinate. This is a no-op for ECDH and ECDSA, which only use the x-coordinate of arbitrary-point operations. Note: ecp_nistp521.c has the same issue in that the documented preconditions are violated by the test case. I have not addressed this in this PR. ecp_nistp521.c does not immediately produce the wrong answer; felem_contract there appears to be a bit more tolerant than its documented preconditions. However, I haven't checked the point_add property above holds. ecp_nistp521.c should either get this same fix, to be conservative, or have the bounds analysi...
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