RSX: sanity check m_alpha_func

Add sanity check for non-zero m_alpha_func and fix glAlphaFunc: opengl error 0x0500 in Resident Evil 4 .

RSX: W RSXThreadNew FBO (1280x720)
RSX: E RSXThreadm_alpha_func = 0x0 , m_alpha_ref = 0.00
RSX: E RSXThreadglAlphaFunc: opengl error 0x0500
RSX: W RSXThreadRSX thread aborted