gdb.base/fileio.c: Fix several -Wmaybe-uninitialized warnings

gdb.base/fileio.c: Fix several -Wmaybe-uninitialized warnings

 src/gdb/testsuite/gdb.base/fileio.c: In function ‘test_write’:
 src/gdb/testsuite/gdb.base/fileio.c:158:5: warning: ‘ret’ may be used uninitialized in this function [-Wmaybe-uninitialized]
      printf ("write 1: ret = %d, errno = %d\n", ret, errno);
      ^

gdb/ChangeLog:
2017-05-18  Pedro Alves  <palves@redhat.com>

	* gdb.base/fileio.c (test_write, test_read, test_close)
	(test_fstat): Don't print 'ret' in the fail path.