One of the assertions at the start of _mongoc_buffer_append is as follows:
The check of (buffer->datalen + data_size) is in the wrong place. Logically, it ensures that the allocated size of the buffer (not the actual utilized storage within the allocation) can be increased without overflowing INT_MAX. This check is only needed just before a reallocation of the buffer into a larger memory area.
The check in its current place at the entry to the function is incorrect because upon entering the function after a reallocation of the buffer to a size of INT_MAX the assertion will fail, even when the actual buffer contents only occupy slightly more than INT_MAX/2.
Other append functions nearby to _mongoc_buffer_append contain the same logic defect.
The correct behavior is for the assertion to occur immediately preceding the call to bson_next_power_of_two.