Speed up reading application metadata block

I used to fill the buffer with zeros before, but they would be
overwritten later anyway. This is not such an issue, except that filling
this buffer with zeros by using an iterator, is apparently terribly
inefficient.

The performance difference in debug mode is as much as a factor 1000.
From ~2.3 seconds to 0.0025 seconds. In release mode this does not
matter so much.

This issue was discovered as a slow case using libfuzzer and cargo-fuzz.

src/metadata.rs

@@ -7,7 +7,6 @@
 
 //! The `metadata` module deals with metadata at the beginning of a FLAC stream.
 
-use std::iter;
 use error::{Result, fmt_err};
 use input::ReadBytes;
 
@@ -268,7 +267,12 @@ fn read_application_block<R: ReadBytes>(input: &mut R, length: u32) -> Result<(u
  let id = try!(input.read_be_u32());
 
  // Four bytes of the block have been used for the ID, the rest is payload.
- let mut data: Vec<u8> = iter::repeat(0).take(length as usize - 4).collect();
+ // Create a vector of uninitialized memory, and read the block into it. The
+ // uninitialized memory is never exposed: read_into will either fill the
+ // buffer completely, or return an err, in which case the memory is not
+ // exposed.
+ let mut data = Vec::with_capacity(length as usize - 4);
+ unsafe { data.set_len(length as usize - 4); }
  try!(input.read_into(&mut data));
 
  Ok((id, data))