avutil/ppc/cpu: Our code assumes vsx implies altivec, thus assert this

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/ppc/cpu.c

@@ -96,6 +96,8 @@ int ff_get_cpu_flags_ppc(void)
                 if (buf[i + 1] & PPC_FEATURE_HAS_POWER8)
                     ret |= AV_CPU_FLAG_POWER8;
 #endif
+                if (ret & AV_CPU_FLAG_VSX)
+                    av_assert0(ret & AV_CPU_FLAG_ALTIVEC);
                 goto out;
             }
         }