More cleanups for input/buffers code

When calling xmlParserInputBufferPush, the buffer may be reallocated
and at the input level the pointers for base, cur and end need to
be reevaluated.
* buf.c buf.h: add two new functions, one to get the base from the
  input of the buffer, and another one to reset the pointers based
  on the cur and base inded
* HTMLparser.c parser.c: cleanup to use the new helper functions
  as well as making sure size_t is used for the indexes computations