Panic on malformed input: attempt to divide by zero

[Shnatsel]

With #32 applied which fixed the early panics fuzzer could further into the decoding code. Here are some samples discovered by AFL that trigger a panic: divide-by-zero.zip

Backtrace:

thread 'main' panicked at 'attempt to divide by zero', /home/shnatsel/Code/image-tiff/src/decoder/mod.rs:539:12
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:211
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:227
   4: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:491
   5: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:398
   6: rust_begin_unwind
             at src/libstd/panicking.rs:325
   7: core::panicking::panic_fmt
             at src/libcore/panicking.rs:95
   8: core::panicking::panic
             at src/libcore/panicking.rs:59
   9: <tiff::decoder::Decoder<R>>::read_image
  10: std::panicking::try::do_call
  11: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:102
  12: afl::read_stdio_bytes
  13: std::rt::lang_start::{{closure}}
  14: std::panicking::try::do_call
             at src/libstd/rt.rs:59
             at src/libstd/panicking.rs:310
  15: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:102
  16: std::rt::lang_start_internal
             at src/libstd/panicking.rs:289
             at src/libstd/panic.rs:398
             at src/libstd/rt.rs:58
  17: main
  18: __libc_start_main
  19: _start

Steps to reproduce are the same as in #28