regcomp fails with custom malloc

[Robert Vollmert]

regcomp fails to compile the regular expression "^ " with memory error if malloc(0) returns NULL. That's valid malloc behavior, compare the man page malloc(3).

I had a little trouble debugging, but I believe the problem lies with posix/regex_internal.c:re_node_set_alloc being called with size == 0, and then treating the zero return value as an error.

I'm attaching a test program that should exhibit the bug.

ETA: I've tested this with libc6_2.15-0ubuntu10_amd64 under VMWare.