TestNil checks for panic behavior for all top-level functions that
accept messages to ensure that we can detect when behavior changes
whether accidentally or deliberately.
This test discovered that the pure protobuf reflect behavior
differs with the fast-path implementation for a few cases.
For the protobuf reflection implementation, we explicitly check
for invalid messages in Merge and Reset. Previously, these would
not panic if the message was already empty (i.e., had no need to
actually set/clear any fields in the receiver message).
Change-Id: I571c0a819366bae993ce7c99b05fb4707e55cd3e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/220958
Reviewed-by: Damien Neil <dneil@google.com>
