geninfo: Ignore empty .gcno files with --initial

Some versions of GCC create empty .gcno files which cause geninfo
to abort processing with an error message:

  geninfo: ERROR: dummy.gcno: reached unexpected end of file

Fix this problem by skipping empty .gcno files.

Reported-by: Maarten Hoes <hoes.maarten@gmail.com>
Signed-off-by: Peter Oberparleiter <oberpar@linux.vnet.ibm.com>