Fix WSAStartup return value check.

SOCKET_ERROR was not a valid return value.
WSAStartup returns 0 on success, so check that instead.
diff --git a/library/net.c b/library/net.c
index be30fbd..0866149 100644
--- a/library/net.c
+++ b/library/net.c
@@ -146,7 +146,7 @@
 
     if( wsa_init_done == 0 )
     {
-        if( WSAStartup( MAKEWORD(2,0), &wsaData ) == SOCKET_ERROR )
+        if( WSAStartup( MAKEWORD(2,0), &wsaData ) != 0 )
             return( POLARSSL_ERR_NET_SOCKET_FAILED );
 
         wsa_init_done = 1;