Add unconditional return statement to yuva420_rgb32_MMX() / yuva420_bgr32_MMX().

Add unconditional return statement to yuva420_rgb32_MMX() / yuva420_bgr32_MMX().

When HAVE_7REGS was not defined these functions had an empty body
causing the following warnings during compilation.
In file included from libswscale/x86/yuv2rgb_mmx.c:58:
libswscale/x86/yuv2rgb_template.c: In function ‘yuva420_rgb32_MMX’:
libswscale/x86/yuv2rgb_template.c:412: warning: no return statement in function returning non-void
libswscale/x86/yuv2rgb_template.c: In function ‘yuva420_bgr32_MMX’:
libswscale/x86/yuv2rgb_template.c:457: warning: no return statement in function returning non-void

Signed-off-by: Diego Biurrun <diego@biurrun.de>