util: fix va_start usage bug

Detected by clang.  POSIX requires that the second argument to
va_start be the name of the last variable; and in some implementations,
passing *path instead of path would dereference bogus memory instead
of pulling arguments off the stack.

* src/util/util.c (virBuildPathInternal): Use correct argument to
va_start.

src/util/util.c

@@ -2799,7 +2799,7 @@ int virBuildPathInternal(char **path, ...)
     va_list ap;
     int ret = 0;
 
-    va_start(ap, *path);
+    va_start(ap, path);
 
     path_component = va_arg(ap, char *);
     virBufferAdd(&buf, path_component, -1);