regex: don't deref NULL upon heap allocation failure

regex: don't deref NULL upon heap allocation failure

* posix/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+++++++++++++++++++++ )