Generate just one error code if iterated SSL_CTX_get() fails.

Avoid enabled 'assert()' in production library.

doc/crypto/BN_CTX_new.pod

@@ -27,6 +27,10 @@ B<BN_CTX>.
 
 BN_CTX_free() frees the components of the B<BN_CTX>, and if it was
 created by BN_CTX_new(), also the structure itself.
+If L<BN_CTX_start(3)|BN_CTX_start(3)> has been used on the B<BN_CTX>,
+L<BN_CTX_end(3)|BN_CTX_end(3)> must be called before the B<BN_CTX>
+may be freed by BN_CTX_free().
+
 
 =head1 RETURN VALUES
 

doc/crypto/BN_CTX_start.pod

@@ -36,7 +36,9 @@ BN_CTX_start() and BN_CTX_end() return no values.
 BN_CTX_get() returns a pointer to the B<BIGNUM>, or B<NULL> on error.
 Once BN_CTX_get() has failed, the subsequent calls will return B<NULL>
 as well, so it is sufficient to check the return value of the last
-BN_CTX_get() call.
+BN_CTX_get() call. In case of an error, an error code is set, which
+can be obtained by L<ERR_get_error(3)|ERR_get_error(3)>.
+
 
 =head1 SEE ALSO