Calculate scale for fit crashes when no images present

[gurgalex]

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    Finished dev [unoptimized + debuginfo] target(s) in 0.05s
     Running `target/debug/riv d`
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:345:21
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:70
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:58
             at src/libstd/panicking.rs:200
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:215
   4: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:478
   5: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:385
   6: rust_begin_unwind
             at src/libstd/panicking.rs:312
   7: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
   8: core::panicking::panic
             at src/libcore/panicking.rs:49
   9: <core::option::Option<T>>::unwrap
             at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/libcore/macros.rs:11
  10: riv::program::Program::calculate_scale_for_fit
             at src/program/mod.rs:140
  11: riv::program::Program::toggle_fit
             at src/program/mod.rs:126
  12: riv::program::Program::run_normal_mode
             at src/program/mod.rs:481
  13: riv::program::Program::run
             at src/program/mod.rs:443
  14: riv::main
             at src/main.rs:41
  15: std::rt::lang_start::{{closure}}
             at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/libstd/rt.rs:64
  16: std::panicking::try::do_call
             at src/libstd/rt.rs:49
             at src/libstd/panicking.rs:297
  17: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:87
  18: std::rt::lang_start_internal
             at src/libstd/panicking.rs:276
             at src/libstd/panic.rs:388
             at src/libstd/rt.rs:48
  19: std::rt::lang_start
             at /rustc/6c2484dc3c532c052f159264e970278d8b77cdc9/src/libstd/rt.rs:64
  20: main
  21: __libc_start_main
  22: _start