Merge #578: Avoid implementation-defined and undefined behavior when dealing with...

14c7dbd Simplify control flow in DER parsing (Tim Ruffing)
ec8f20b Avoid out-of-bound pointers and integer overflows in size comparisons (Tim Ruffing)
01ee1b3 Parse DER-enconded length into a size_t instead of an int (Tim Ruffing)
3cb057f Fix possible integer overflow in DER parsing (Tim Ruffing)

Pull request description:

  This is a result of auditing the code for overflow issues at random places. None of this is critical but I think all of it should be fixed.

  I know this touches "red" code. I double-checked and triple-checked this but I can understand if some of the changes are not desirable because they change well-tested code.

  Best reviewed in individual commits.

ACKs for commit 14c7db:

Tree-SHA512: 312dd3f961739752e1a861e75bd755920f634f87ee9668793e102c224434e8d21367452e114de729322c71a89f4fa82126aa5d32742f2bbbc091777c99515e10