Refine read_string

Refine read_string

In read_string, we have this line

  chunksize = (len == -1 ? min (8, fetchlimit) : fetchlimit);

but chunksize is only used in the block that lne == -1, so IWBN to
move chunksize to the block in which it is used, and simplify the
condition setting chunksize.  This patch also moves 'found_nul' to
inner block.  This patch also splits a paragraph of comment into two,
and move them to different condition blocks (len > 0 and len == -1)
respectively.

gdb:

2014-11-23  Yao Qi  <yao@codesourcery.com>

	* valprint.c (read_string): Move local variables 'found_nul',
	'chunksize' and 'limit' to inner scope.  Update comments.