Zero out high bytes of 80-bit ldnan

Zero out high bytes of 80-bit ldnan

1. Backport of dd6885360b15e763e0706dc28555eea328de432d

   fix garbage filtering with 80-bit long doubles

   The test had several problems that resulted in the excess
   bytes not getting zeroed out. This caused random contents in
   $Config{longdblinfbytes}, observed on Debian with GCC 6.2.0 (but not
   5.4.1).

   Bug-Debian: https://bugs.debian.org/844752

2. Backport of 6b2c7479d6d33249c2385d8a36d8ee9e61e1a519

   also zero out high bytes of 80-bit ldnan

   These are currently zero anyway, but things are probably not guaranteed
   to stay so.