Fix potential capacity overflow when encoding with fixed blocks

Currently, this crate always tries to use dynamic DEFLATE blocks, so the
codepath for static blocks is mostly unused. I've fixed it to use static
blocks in a working copy instead, and discovered that empty files, where
instart == inend, may trigger a bad allocation in that codepath.

src/squeeze.rs

@@ -8,7 +8,7 @@
 //! solution.
 
 use log::{debug, trace};
-use std::{cmp, f32, f64};
+use std::{cmp, f64};
 
 use crate::cache::Cache;
 use crate::deflate::{calculate_block_size, BlockType};
@@ -447,7 +447,7 @@ pub fn lz77_optimal_fixed<C>(
     s.blockstart = instart;
     s.blockend = inend;
     let mut h = ZopfliHash::new();
-    let mut costs = Vec::with_capacity(inend - instart - 1);
+    let mut costs = Vec::with_capacity(inend - instart);
     lz77_optimal_run(
         s,
         in_data,