Reject hexadecimal blobs with odd number of characters, fixes #12650.

Changes in 4e75f84 to allow ignoring
non-hexa characters (such as whitespace) have also allowed dangling
hex digits, which were then padded with an implicit 0. This restores
the original behaviour of throwing an exception for these inputs.

pdns/rcpgenerator.cc

@@ -546,8 +546,7 @@ static void HEXDecode(const char* begin, const char* end, string& out)
     }
   }
   if(mode)
-    out.append(1, (char) val);
-
+    throw RecordTextException("Hexadecimal blob with odd number of characters");
 }
 
 void RecordTextReader::xfrHexBlob(string& val, bool keepReading)