der: pem reader rejects zero lengths reads

Since base64ct 1.7.0 ([RustCrypto#1387]), it now rejects zero lengths reads. This 
happens to trigger with 


[RustCrypto#1387]: RustCrypto#1387

Author: baloo

Cargo.lock

@@ -87,6 +87,10 @@ impl<'i> Reader<'i> for PemReader<'i> {
     }
 
     fn read_into<'o>(&mut self, buf: &'o mut [u8]) -> crate::Result<&'o [u8]> {
+        if buf.is_empty() {
+            return Ok(buf);
+        }
+
         let new_position = (self.position + buf.len())?;
         if new_position > self.input_len {
             return Err(ErrorKind::Incomplete {