vcs-svn: rename check_overflow and its arguments for clarity

vcs-svn: rename check_overflow and its arguments for clarity

The canonical interpretation of a range a,b is as an interval [a,b),
not [a,a+b), so this function taking argument names a and b feels
unnatural.  Use more explicit names "offset" and "len" to make the
arguments' type and function clearer.

While at it, rename the function to convey that we are making sure
the sum of this offset and length do not overflow an off_t, not a
size_t.

[jn: split out from a patch from Ramsay Jones, then improved with
 advice from Thomas Rast, Dmitry Ivankov, and David Barr]

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Improved-by: Dmitry Ivankov <divanorama@gmail.com>