diffie-hellman: Explicitly initialize DH exponent sizes during initialization

To avoid any race conditions when multiple threads call and initialize
diffie_hellman_get_params(), explicitly examine the optimum DH exponent size
during library initialization.

Fixes #655.

src/libstrongswan/library.c

@@ -351,5 +351,7 @@ bool library_init(char *settings, const char *namespace)
 #endif /* INTEGRITY_TEST */
 	}
 
+	diffie_hellman_init();
+
 	return !this->integrity_failed;
 }