bug#20957: 2.21 bug in handling at least one -P regular expression

[vampyrebat]

grep 2.21 incorrectly handles a -P regular expression that 2.20 handled 
correctly.  Thanks for looking into this.

$ cat file
Here's a line.

This line has one blank line above it.


This line has two blank lines above it.



This line has three blank lines above it.




This line has four blank lines above it.

$ grep-2.20/src/grep -Pzo '(?<=\n\n\n).*' file
This line has two blank lines above it.
This line has three blank lines above it.
This line has four blank lines above it.
$ grep-2.21/src/grep -Pzo '(?<=\n\n\n).*' file
This line has two blank lines above it.