Fix a bug due to the operator precedence of C.

The expression ```! pic_length(pic, spec) == 3``` is treated as ```(!
pic_length(pic, spec)) == 3``` since ! has higher precedence than ==.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Merge pull request #41 from omasanori/miscompare

Fix a bug due to the operator precedence of C.