e2fsck: Explicitly reject extents that begin at physical block 0 as illegal

In the case where s_first_data_block is 1, we need to explictly reject
an extent whose starting physical block is zero.

Thanks to Jiaying Zhang <jiayingz@google.com> for finding this bug.

Addresses-Google-Bug: #2573806

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
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