Merge #199: hrp: small improvement in hrp parsing

3322ca4 hrp: small improvement in hrp parsing (Thomas Coratger)

Pull request description:

  In the HRP parsing function, we first test the following conditions with potential error outputs:

  ```rust
  if hrp.is_empty() {
      return Err(Empty);
  }
  if hrp.len() > MAX_HRP_LEN {
      return Err(TooLong(hrp.len()));
  }
  ```

  So we don't need to directly declare the `new` variable in case an error is thrown and this declaration becoming useless. We can wait for the checks to be ok before declaring this (very small improvement).

ACKs for top commit:
  tcharding:
    ACK 3322ca4
  apoelstra:
    ACK 3322ca4 successfully ran local tests
  clarkmoody:
    ACK 3322ca4

Tree-SHA512: 92da994c5c504cf34e443646548a567076edb57fcbb88e08bada8d1e60ffaeaced50e5665b22dfded5c31107671d3ca7f49dc6eaaeb732a43ac424a3425fa145

src/primitives/hrp.rs

@@ -78,15 +78,15 @@ impl Hrp {
     pub fn parse(hrp: &str) -> Result<Self, Error> {
         use Error::*;
 
-        let mut new = Hrp { buf: [0_u8; MAX_HRP_LEN], size: 0 };
-
         if hrp.is_empty() {
             return Err(Empty);
         }
         if hrp.len() > MAX_HRP_LEN {
             return Err(TooLong(hrp.len()));
         }
 
+        let mut new = Hrp { buf: [0_u8; MAX_HRP_LEN], size: 0 };
+
         let mut has_lower: bool = false;
         let mut has_upper: bool = false;
         for (i, c) in hrp.chars().enumerate() {