Don't reference past the end of the vector

Don't reference past the end of the vector

An earlier change made find_source_lines read:

    end = &data[size];

However, since 'size' is the size of the vector, this seems fishy.
More obviously ok is to compute the end of the data directly:

    end = data.data () + size;

2018-02-09  Tom Tromey  <tom@tromey.com>

	* source.c (find_source_lines): Don't reference past the end of
	the vector.