libdm: fix uninitialized variable warnings on older gcc

libdm: fix uninitialized variable warnings on older gcc

Older versions of gcc aren't able to track the assignments of
local variables as well as the latest versions leading to spurious
warnings like:

libdm-stats.c:2183: warning: "len" may be used uninitialized in this
function
libdm-stats.c:2177: warning: "minwidth" may be used uninitialized in
this function

Both of these variables are in fact assigned in all possible paths
through the function and later compilers do not produce these
warnings.

There's no reason to not initialize these variables though and
it makes the function slightly easier to follow.

Also fix one use of 'unsigned' for a nr_bins value.