Rewrite NativeBN_twosCompFitsIntoBytes.

valgrind complains about invalid 4-byte reads, caused by "case 8" in the
BNInterface.c function I'm removing here, which assumed that if we're checking
whether a BIGNUM fits in 8 bytes, it must require more than 4 bytes (and so
accessing d[1] is acceptable).

We can implement this in Java using the existing BigInteger.bitLength
method (which may call down to native code, but that native code looks okay).

Also remove a related commented-out method.

Bugs: 2223213, 2225642
3 files changed