Fix bug in tag overflow checks

[marcoferrer]

msg.Unmarshal() returns variable length integer overflow when decoding proto extension with a field number below the protowire.MaxValidNumber const. This is because the tag.Overflows() check incorrectly calculated the max number of leading zeros for the field number

Issue with more context: #18

[mcy]

Please expand the PR description to detail why this change is necessary. I prefer to have detailed commit messages that are self-contained.

[mcy]

Also, could you include a test in internal/testdata/tags that exercises this behavior? I like to have end-to-end tests for these kinds of corner-cases.