BUG/MEDIUM: jwt: Clear SSL error queue on error when checking the signature

BUG/MEDIUM: jwt: Clear SSL error queue on error when checking the signature

When the signature included in a JWT is verified, if an error occurred, one
or more SSL errors are queued and never cleared. These errors may be then
caught by the SSL stack and a fatal SSL error may be erroneously reported
during a SSL received or send.

So we must take care to clear the SSL error queue when the signature
verification failed.

This patch should fix issue #2643. It must be backported as far as 2.6.

(cherry picked from commit 46b1fec0e9a6afe2c12fd4dff7c8a0d788aa6dd4)
Signed-off-by: Willy Tarreau <w@1wt.eu>