Added python3 compatibility string conversion to ubivalid.py plugin

src/binwalk/plugins/ubivalid.py

 import struct
 import binascii
 import binwalk.core.plugin
-
+import binwalk.core.compat
 
 class UBIValidPlugin(binwalk.core.plugin.Plugin):
     '''
@@ -56,7 +56,7 @@ class UBIValidPlugin(binwalk.core.plugin.Plugin):
             # Seek to and read the suspected UBI erase count header
             fd = self.module.config.open_file(result.file.name, offset=result.offset)
 
-            ec_header = fd.read(1024)
+            ec_header = binwalk.core.compat.str2bytes(fd.read(1024))
             fd.close()
 
             result.valid = self._check_crc(ec_header[0:64])