Fix test in anonymous_struct_prefix

Fix test in anonymous_struct_prefix

I noticed a bad test in dwarf2/read.c:anonymous_struct_prefix:

   attr = dw2_linkage_name_attr (die, cu);
   const char *attr_name = attr->as_string ();
  if (attr == NULL || attr_name == NULL)
    return NULL;

Here, if attr==NULL, this will crash before the test can be executed.

This patch fixes the problem by hoisting the test.  I'm checking this
in as obvious.