Fix infinite loop on a truncated RFC4176 public key file.

You could reproduce this, for example, by cutting the final line
reading "---- END SSH2 PUBLIC KEY ----" off the end of a file, and
feeding it to Unix 'puttygen -l'.

rfc4716_loadpub() was looping round on get_chomped_line() until it
found a line starting with "-" after the base64 data. But it failed to
check for the end of the file as well, so if the data was truncated,
it would just keep spinning at the end of the file.

sshpubk.c

@@ -1096,6 +1096,8 @@ static bool rfc4716_loadpub(BinarySource *src, char **algorithm,
             }
         }
         sfree(line); line = NULL;
+        if (!get_avail(src))
+            break;
         line = mkstr(get_chomped_line(src));
     }