If we have to realloc() "list" we need to zero out the new entries

If we have to realloc() "list" we need to zero out the new entries
to match the behavior of the initial calloc().  The "cols" array
does not actually need to be zeroed so use reallocarray() instead
of calloc().  Also fix a memory leak of tbl and initialize the value
of maxcols/maxentry in the assignment instead of in the ecalloc()
call.  OK espie@