Some implementations of sscanf do not handle a space before a trailing %n
properly.

As an example, MSVC's does this for the second insatnce in this patch, for
an input of "0x3:c0=c1:c1=c0":
    1) Match the final "c0" or "c1".
    2) Realize it's at the end of the string.
    3) Check for %n.
    4) There is no %n, but a space instead.
    5) Leave 'len' unitilialized.

So, move it out of the sscanf format strings, and call skip_spaces instead.

This bug does not affect skip_spaces since %n is the first and only formatting
string.
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>