less panicy read_bool

.travis.yml

@@ -8,5 +8,5 @@ matrix:
     - rust: nightly
   fast_finish: true
 script:
-  - cargo build --verbose --all
-  - cargo test --verbose --all
+  - cargo test --all
+  - cargo test --all --all-features

src/buffer.rs

@@ -174,7 +174,7 @@ where
             .get(byte_index)
             .ok_or_else(|| ReadError::NotEnoughData {
                 requested: 1,
-                bits_left: self.bit_len() - position,
+                bits_left: self.bit_len().saturating_sub(position),
             })
             .map(|byte| {
                 let shifted = byte >> bit_offset;