Make room for null terminators in iterator_test

In two locations we assign strings of length 3 to buffers of length 3 when we
need buffers of length 4 to accomodate the \0 at the end of the string.

Signed-off-by: Casey Dahlin <casey.dahlin@gmail.com>
Add gcov make target

Signed-off-by: Casey Dahlin <casey.dahlin@gmail.com>
Quiet spurious compiler warning

Signed-off-by: Casey Dahlin <casey.dahlin@gmail.com>