libsnapshot: op_count_max default to max blocks

In the case that op_count_max is read in as zero, we should use the
upper bound of max blocks as the estimation. One case in which this
error can happen is if a v2 cow estimator is used, we should still be
able to run an OTA if we upper bound our ops buffer size estimation.

Test: th
Change-Id: I97ca66368d6631bf43c8911ed66f99c9e8096e2d