gallium/util: make sure cache line size is not zero

The "normal" detection (querying clflush size) already made sure it is
non-zero, however another method did not. This lead to crashes if this
value happened to be zero (apparently can happen in virtualized environments
at least).
This fixes https://bugs.freedesktop.org/show_bug.cgi?id=87913

Cc: "10.4" <mesa-stable@lists.freedesktop.org>

src/gallium/auxiliary/util/u_cpu_detect.c

@@ -409,8 +409,12 @@ util_cpu_detect(void)
       }
 
       if (regs[0] >= 0x80000006) {
+         /* should we really do this if the clflush size above worked? */
+         unsigned int cacheline;
          cpuid(0x80000006, regs2);
-         util_cpu_caps.cacheline = regs2[2] & 0xFF;
+         cacheline = regs2[2] & 0xFF;
+         if (cacheline > 0)
+            util_cpu_caps.cacheline = cacheline;
       }
 
       if (!util_cpu_caps.has_sse) {