Fix crash in lexer refill (reported by Agostino Sarubbo).

The crash happened in a rare case of a very long lexeme that doen't fit
into the buffer, forcing buffer reallocation.

The crash was caused by an incorrect calculation of the shift offset
(it was smaller than necessary). As a consequence, the data from buffer
start and up to the beginning of the current lexeme was not discarded
(as it should have been), resulting in less free space for new data than
expected.