bug#19420: intermittent segfault using grep -P

[Norihiro Tanaka]

In addition, please recompile grep with `-g' and if it is reproduced,
get backtrace from a core file with gdb and send it to us.

  $ CFLAGS=-g ./configure OPTIONS
  $ make
  $ cat grepTxt | grep -P 'b(?:(?!tom).)+'
  $ gdb src/grep core.*

  (gdb) bt