shred: avoid data write pass with --size=0

* src/shred.c (dopass): Exit early to avoid redundant heap
allocation, and more importantly avoiding a file sync
when we're writting no data, as this can have side effects.
Also with --verbose, this avoids printing of "pass status"
which could be confusing as to whether data was actually written.
* tests/misc/shred-passes.sh: Ensure the status for data
passes are not written when not doing any data writes.

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