indexer: Re-use memory buffer when reading RPM repo data

This avoids reading the entire uncompressed XML into a buffer.

indexer/src/sys_rpm.rs

@@ -73,6 +73,7 @@ fn readpkgs<F>(url: String, mut cb: F) -> Result<(),Box<Error>>
     let arch_src = Some("src".to_string());
 
     loop {
+        buf.clear();
         let event = xml.read_event(&mut buf);
         let event = event.map_err(to_err)?;
 
@@ -141,6 +142,7 @@ fn repomd(url: String) -> Result<(String,String),Box<Error>> {
     let mut buf = Vec::new();
 
     loop {
+        buf.clear();
         let event = xml.read_event(&mut buf).map_err(to_err)?;
         match event {
             Event::Start(ref e) |