If y is zero, there is no point with odd y, so the odd bit may not be
set, hence EC_R_INVALID_COMPRESSION_BIT. This code instead computed the
Kronecker symbol of x and changed the error code to
EC_R_INVALID_COMPRESSED_POINT if not a square.
As the comment says, this was (intended to be) unreachable. But it
seems x was a typo for tmp1. It dates to before upstream's
6fb60a84dd1ec81953917e0444dab50186617432, when BN_mod_sqrt gave
garbage if its input was not square. Now it emits BN_R_NOT_A_SQUARE.
Upstream's 48fe4d6233ac2d60745742a27f820dd88bc6689d then mapped
BN_R_NOT_A_SQUARE to EC_R_INVALID_COMPRESSED_POINT.
Change-Id: Id9e02fa1c154b61cc0c3a768c9cfe6bd9674c378
Reviewed-on: https://boringssl-review.googlesource.com/12463
Reviewed-by: Adam Langley <agl@google.com>
14ebb4ff27