MINOR: sample: make sample_parse_expr() able to return an end pointer

MINOR: sample: make sample_parse_expr() able to return an end pointer

When an end pointer is passed, instead of complaining that a comma is
missing after a keyword, sample_parse_expr() will silently return the
pointer to the current location into this return pointer so that the
caller can continue its parsing. This will be used by more complex
expressions which embed sample expressions, and may even permit to
embed sample expressions into arguments of other expressions.