The current warning and/or trap when the btstack is overrun in
dtSearch or dtReadFirst are not very helpful.  Add code to detect
the stack overrun earlier, print something useful, and return
gracefully.

I've found that dbFree being called with blkno == 0 can lead to this
error, so I put in a specific check for that.
Signed-off-by: Dave Kleikamp <shaggy@austin.ibm.com>