pipe: follow up cleanup to previous

The commited patch was incomplete.

• add back missing goto retry, noted by jhb
• 'if (error)' -> 'if (error != 0)'
• consistently do:

if (error != 0)

break;

continue;

instead of:

if (error != 0)

break;

else

continue;

This adds some 'continue' uses which are not needed, but line up with the
rest of pipe_write.