Tests fail with "index out of bounds"

[birktj]

Currently the tests fail with an index out of bounds error, this is probably a bug introduced in #26 that could only be discovered with the tests from #23.

running 1 test
test decoder::stream::test::test_packbits ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

     Running target/debug/deps/decode_images-564f01fb7ded3313

running 6 tests
test test_gray_u8 ... ok
test test_rgb_u8 ... ok
test test_string_tags ... ok
test test_gray_u16 ... ok
test test_decode_data ... ok
test test_rgb_u16 ... ok

test result: ok. 6 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out

     Running target/debug/deps/encode_images-b9c0207f836596cd

running 5 tests
test test_gray_u8_roundtrip ... ok
test test_rgb_u8_roundtrip ... ok
test test_gray_u16_roundtrip ... ok
test encode_decode ... ok
test test_rgb_u16_roundtrip ... FAILED

failures:

---- test_rgb_u16_roundtrip stdout ----
thread 'test_rgb_u16_roundtrip' panicked at 'index out of bounds: the len is 19 but the index is 19', /rustc/9fda7c2237db910e41d6a712e9a2139b352e558b/src/libcore/slice/mod.rs:2463:10
note: Run with `RUST_BACKTRACE=1` for a backtrace.


failures:
    test_rgb_u16_roundtrip

test result: FAILED. 4 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out

error: test failed, to rerun pass '--test encode_images'

[birktj]

Looking a bit more into this, I found that the bug seems to come from two places, the first place is that in the test I try to read the image twice and that the decoder dosen't stop it from trying to read more strips than the image has.

I will try to come with a patch that fixes this and a few other bugs as soon as possible.