runtime: don't stat a NULL filename

Noticed in https://gcc.gnu.org/PR86331.
    
    Reviewed-on: https://go-review.googlesource.com/121417

From-SVN: r262234

gcc/go/gofrontend/MERGE

@@ -1,4 +1,4 @@
-baaaf1e0f1e9a54ea2dfe475154c85c83ec03740
+e1fcce0aec27b1f50ac0e736f39f4c806c2a5baa
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.

libgo/runtime/go-caller.c

@@ -116,7 +116,7 @@ __go_get_backtrace_state ()
 	 argv[0] (http://gcc.gnu.org/PR61895).  It would be nice to
 	 have a better check for whether this file is the real
 	 executable.  */
-      if (stat (filename, &s) < 0 || s.st_size < 1024)
+      if (filename != NULL && (stat (filename, &s) < 0 || s.st_size < 1024))
 	filename = NULL;
 
       back_state = backtrace_create_state (filename, 1, error_callback, NULL);