Fix use-after-free error when parsing a corrupt nested archive.

	PR 21787
	* archive.c (bfd_generic_archive_p): If the bfd does not have the
	correct magic bytes at the start, set the error to wrong format
	and clear the format selector before returning NULL.

bfd/ChangeLog

@@ -1,3 +1,10 @@
+2017-07-19  Nick Clifton  <nickc@redhat.com>
+
+	PR 21787
+	* archive.c (bfd_generic_archive_p): If the bfd does not have the
+	correct magic bytes at the start, set the error to wrong format
+	and clear the format selector before returning NULL.
+
 2017-07-19  Nick Clifton  <nickc@redhat.com>
 
 	PR 21786

bfd/archive.c

@@ -834,7 +834,12 @@ bfd_generic_archive_p (bfd *abfd)
   if (strncmp (armag, ARMAG, SARMAG) != 0
       && strncmp (armag, ARMAGB, SARMAG) != 0
       && ! bfd_is_thin_archive (abfd))
-    return NULL;
+    {
+      bfd_set_error (bfd_error_wrong_format);
+      if (abfd->format == bfd_archive)
+	abfd->format = bfd_unknown;
+      return NULL;
+    }
 
   tdata_hold = bfd_ardata (abfd);