regex: don't deref NULL upon heap allocation failure

* lib/regcomp.c (parse_dup_op): Handle duplicate_tree
failure in one more place.
To trigger the segfault, configure grep -with-included-regex,
build it, and run these commands:
( ulimit -v 300000; echo a|src/grep -E a+++++++++++++++++++++ )
I discovered this while replying to a private report from
Jens Schleusener about excessive memory consumption by grep
when using a regular expression like the one above.

0 files changed, 0 insertions, 0 deletions