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>

pw_i2c/initiator_mock.cc

diff --git a/pw_i2c/initiator_mock.cc b/pw_i2c/initiator_mock.cc
index 45dd340..029317e 100644
--- a/pw_i2c/initiator_mock.cc
+++ b/pw_i2c/initiator_mock.cc
@@ -44,7 +44,7 @@
 
   ConstByteSpan expected_rx_buffer =
       expected_transactions_[expected_transaction_index_].read_buffer();
-  EXPECT_EQ(expected_rx_buffer.size(), rx_buffer.size());
+  PW_CHECK_INT_EQ(expected_rx_buffer.size(), rx_buffer.size());
 
   std::copy(
       expected_rx_buffer.begin(), expected_rx_buffer.end(), rx_buffer.begin());