Correct expected failure value for call to RSA_padding_add_PKCS1_PSS

Using OpenSSL 1.0.1j, looking at crypto/rsa/rsa_pss.c, function RSA_padding_add_PKCS1_PSS() calls function RSA_padding_add_PKCS1_PSS_mgf1() within the same source file.  The latter function returns 0 for failure or 1 for success, which the former function passes through to its caller.  rsa_padding_add_pkcs1_pss() tests the return of RSA_padding_add_PKCS1_PSS() for -1 as the failure code, which will never occur.  

I hope this is the correct place to submit fixes to M2Crypto.  The page at https://pypi.python.org/pypi/M2Crypto/ says to go to https://github.com/M2Crypto/M2Crypto, whose README says to go back to the pypi.python.org URL to report bugs.  The M2Crypto/M2Crypto page also references http://chandlerproject.org/Projects/MeTooCrypto, which says that this repo maintains the code for M2Crypto and its owner is the maintainer.