This causes MALLOC_DEBUG reporting to be slightly different, calls which
cause additional memory pools to be allocated now report the callers
location rather than the location which originally allocated the
string field structure.  This reduces storage needed by string fields
and allows MALLOC_DEBUG to identify the source of additional allocations
rather than obscuring it by reporting the original allocation caller.

Change-Id: Idd18e6639a87ab862079b580c114d90361412289