video: vidconsole: Fix null dereference of ops->measure

At present vidconsole_measure() tests ops->select_font before calling
ops->measure, which would result in a null dereference when the console
driver provides no ops for measure.

Fixes: b828ed7d ("console: Allow measuring the bounding box of text")
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Simon Glass <sjg@chromium.org>