Fixes for commit 211dd1e9

Fixes for commit 211dd1e9

Fixes for issues in commit 211dd1e9 noted by by Jim Meyering.

1. Allocate content buffer of size content_length + 1 to ensure
   NUL-termination.
2. Limit content buffer size to 64k
3. Fix whitespace issue

V2:
  - Add comment to clarify allocation of content buffer
  - Add ATTRIBUTE_NONNULL where appropriate
  - User NULLSTR macro