cr50: avoid infinite looping w/ out of range inputs
cr50: avoid infinite looping w/ out of range inputs

Make the dcrypto ecdsa verify code check that r,s are in range, and
not depend on the caller C code to have done so.
For instance, s equal to 0 would result in infinite loop during
computation of its modular inverse.

BRANCH=none
BUG=b:35587381
TEST=TCG tests pass
Change-Id: I13f7811be030aed9feaa11c45dc68d4bfd08fb76
Reviewed-on: https://chromium-review.googlesource.com/508819
Commit-Ready: Marius Schilder <d423f2980be0298efdef7df239b147d21d472f26@chromium.org>
Tested-by: Marius Schilder <d423f2980be0298efdef7df239b147d21d472f26@chromium.org>
Reviewed-by: Marius Schilder <d423f2980be0298efdef7df239b147d21d472f26@chromium.org>
Reviewed-by: Vadim Bendebury <5515d6d2d0829cbe0dd0dcf2094aaded06d58514@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec