`zeroes(1,1,0) & zeroes(1,1,0)` segfaults

[falsifian]

pdl> zeroes(1,1,0) & zeroes(1,1,0)
Segmentation fault (core dumped) 

This is PDL 2.081 on OpenBSD current on amd64.

[mohawk2]

For the sake of posterity: looping in PDL operates at 3 levels:

• there are each operation's "inherent" dimensions (which might be none, for a dimensionless/scalar operation like addition), over which any broadcasting happens; any loops happen within each operation's code
• there are two further dimension that happen in the "inner loop" generated for each readdata/writebackdata
• anything above that gets managed by startbroadcastloop which sets up data structures, and iterbroadcastloop which moves along higher dimensions until done

The two inner loops already knew about dealing with empties, because they are implemented with two for loops, which are a while loop with extra stuff, but that didn't help here because the and2 operation is dimensionless, and the two inner-loop dims had 1 each. The iterbroadcastloop knew how to stop correctly.

However, the startbroadcastloop did not check for empty and do nothing; the commit above changes that, and now all works correctly.