fix unintended dereference (seg fault)

[John-194]

This fixes a rare segmentation fault.

What was happening:
grid.h table = new tableT(cellMax*2, cellHash<dim, objT>(myHash)); passes a temporary cellHash.

original code:

 Table(intT size, HASH hashF) :  
    m((intT)1 << utils::log2Up(100+(intT)(2.0*(float)size))),
    mask(m-1),
    empty(hashF.empty()),  
    hashStruct(hashF),
    TA(newA(eType,m)),
    compactL(NULL),
	  load(2.0)
      { clearA(TA,m,empty);
      }

Problem: hashF is the temporary, empty() returns pointer to cell, which is a part of the tmp. The tmp cell gets deleted after the Table initialization list finishes. hashF (tmp) gets copied to hashStruct via copy constructor, but empty(hashF.empty()) ends up dereferenced.
Fix: Initialize empty from the copy of hashF (hashStruct). This only works after the initialization list has finished.

snippet of ASAN error message:

==6037==ERROR: AddressSanitizer: heap-use-after-free on address 0xffffab64a928 at pc 0xffffa53db448 bp 0xffff935cb950 sp 0xffff935cb960
READ of size 8 at 0xffffab64a928 thread T9
    #0 0xffffa53db444 in point<2>::isEmpty() include/dbscan/point.h:31
    #1 0xffffa53db444 in cell<2, point<2> >::isEmpty() include/dbscan/cell.h:111
    #2 0xffffa53db444 in cellHash<2, point<2> >::cmp(cell<2, point<2> >, cell<2, point<2> >) include/dbscan/cell.h:166
    #3 0xffffa53db444 in Table<cellHash<2, point<2> >, int>::insert(cell<2, point<2> >) include/dbscan/pbbs/ndHash.h:134
    #4 0xffffa53db444 in grid<2, point<2> >::insertParallel(point<2>, point<2>, int, int, int)::{lambda(int)#5}::operator()(int) const include/dbscan/grid.h:232
    #5 0xffffa5402438 in void parlay::fork_joinscheduler::parfor<grid<2, point<2> >::insertParallel(point<2>, point<2>, int, int, int)::{lambda(int)#5}>(unsigned long, unsigned long, grid<2, point<2> >::insertParallel(point<2>, point<2>, int, int, int)::{lambda(int)#5}, unsigned long, bool) include/dbscan/pbbs/scheduler.h:373

[skbill84]

Fantastic catch! Thanks so much for digging into this. Really appreciate your thoroughness here.