Don't allow zero clues in Pattern

Some nonogram implementations allow zero clues so that a row or column
with a single zero clue is equivalent to one with no clues, that is it
has no black squares in it.  Pattern, however, doesn't interpret them
like this and treats a puzzle with a zero clue as insoluble, so it's
not helpful to permit them.

Permitting zero clues also confuses Pattern's memory allocation so
that it can suffer a buffer overrun.  As an example, before this
commit a build with AddressSanitizer would report a buffer overrun
with the description "1:0/0.0" because it tries to put two clues in a
row that can have a maximum of one.

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