Missing checks when malloc returns NULL...

Hi all,

While auditing the code, I got surprised that there are a couple of
code paths that do nothing for this error handling:
- pg_regress and isolationtester use malloc extensively, in case of
failure those would just crash crash. I think that it matters for
buildfarm members that are under memory pressure to not do so, so
those should use pg_malloc instead.
- refint.c makes use of malloc to store plans in top memory context.
That's a buggy concept clearly... This code would need to be reworked
more largely than in the patch I attach.
- pg_dlsym for darwin uses malloc, but would crash on failure
- ps_status.c does nothing when it uses malloc().
- sprompt.c uses malloc once, and would crash on failure
- mcxt.c uses that, which is surprising:
@@ -704,7 +704,8 @@ MemoryContextCreate(NodeTag tag, Size size,
    {
        /* Special case for startup: use good ol' malloc */
        node = (MemoryContext) malloc(needed);
-       Assert(node != NULL);
+       if (node == NULL)
+           elog(PANIC, "out of memory");
    }
I think that a PANIC is cleaner here instead of a simple crash.

So attached is a patch aimed at improving things. Thoughts?
--
Michael