mtlib: Fix a use of `len` on a map object

This was breaking many touch tests when run in Python 3.

BUG=none
TEST=run touchtests and check for related errors

Change-Id: Ia9263d1461d7632e47e30d3a0ec11c7adbd37f63
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/mttools/+/2145031
Tested-by: Harry Cutts <hcutts@chromium.org>
Auto-Submit: Harry Cutts <hcutts@chromium.org>
Reviewed-by: Sean O'Brien <seobrien@chromium.org>
Commit-Queue: Sean O'Brien <seobrien@chromium.org>
1 file changed