swscale/x86/yuv2rgb: fix crashes when loading alpha from unaligned buffers

Regression since fc6a5883d6 on SSSE3 enabled
CPUs.

Fixes ticket #8955.

Signed-off-by: Marton Balint <cus@passwd.hu>

libswscale/x86/yuv_2_rgb.asm

@@ -286,7 +286,7 @@ cglobal %1_420_%2%3, GPR_num, GPR_num, reg_num, parameters
 %ifidn %1, yuv
     pcmpeqd m3, m3 ; Set alpha empty
 %else
-    mova m3, [pa_2indexq + 2 * indexq] ; Load alpha
+    movu m3, [pa_2indexq + 2 * indexq] ; Load alpha
 %endif
     mova m5, m_blue
     mova m6, m_red