OpenSSL: fix spurious SSL connection aborts

Was seeing spurious SSL connection aborts using libcurl and
OpenSSL. I tracked it down to uncleared error state on the
OpenSSL error stack - patch attached deals with that.

Rough idea of problem:

Code that uses libcurl calls some library that uses OpenSSL but
don't clear the OpenSSL error stack after an error.

ssluse.c calls SSL_read which eventually gets an EWOULDBLOCK from
the OS. Returns -1 to indicate an error

ssluse.c calls SSL_get_error. First thing, SSL_get_error calls
ERR_get_error to check the OpenSSL error stack, finds an old
error and returns SSL_ERROR_SSL instead of SSL_ERROR_WANT_READ or
SSL_ERROR_WANT_WRITE.

ssluse.c returns an error and aborts the connection

Solution:

Clear the openssl error stack before calling SSL_* operation if
we're going to call SSL_get_error afterwards.

Notes:

This is much more likely to happen with multi because it's easier
to intersperse other calls to the OpenSSL library in the same
thread.

CHANGES

@@ -6,6 +6,11 @@
 
                                   Changelog
 
+Daniel Stenberg (5 June 2010)
+- Constantine Sapuntzakis fixed a case of spurious SSL connection aborts using
+  libcurl and OpenSSL. "I tracked it down to uncleared error state on the
+  OpenSSL error stack - patch attached deals with that."
+
 Daniel Stenberg (5 June 2010)
 - Frank Meier added CURLINFO_PRIMARY_PORT, CURLINFO_LOCAL_IP and
   CURLINFO_LOCAL_PORT to curl_easy_getinfo().