pw_i2c: Fix invalid copy

Add error checking for copying expected rx data to rx buffer in
MockInitiator to prevent potential seg faults.

Testing:
Create a failed expectation case for reading data.
See the assert instead of seg fault.

Change-Id: I856534b15ecc7f1971af034535264be0f1615fca
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/49481
Reviewed-by: Ewout van Bekkum <ewout@google.com>
Commit-Queue: Kevin Zeng <zengk@google.com>