Bug 3870: assertion failed: String.cc: 'len_ + len <65536' in ESI::CustomParser

Bug 3870: assertion failed: String.cc: 'len_ + len <65536' in ESI::CustomParser

The custom ESI parser used in absence of libxml2 or libexpat parsers was
restricted to handling 64KB buffers but under some conditions could expand
to over 64KB during the parse process. Hitting this assertion.

TODO: the parser can now be redesigned to make use of Tokenizer and
      CharacterSet parsing tools. But that is left for later work.