Implement reallocarray()

Stateless and I stumbled upon this issue while discussing the
semantics of read, accepting a size_t but only being able to return
ssize_t, effectively lacking the ability to report successful
reads > SSIZE_MAX.
The discussion went along and we came to the topic of input-based
memory allocations. Basically, it was possible for the argument
to a memory-allocation-function to overflow, leading to a segfault
later.
The OpenBSD-guys came up with the ingenious reallocarray-function,
and I implemented it as ereallocarray, which automatically returns
on error.
Read more about it here[0].

A simple testcase is this (courtesy to stateless):
$ sbase-strings -n (2^(32|64) / 4)

This will segfault before this patch and properly return an OOM-
situation afterwards (thanks to the overflow-check in reallocarray).

[0]: http://www.openbsd.org/cgi-bin/man.cgi/OpenBSD-current/man3/calloc.3

unexpand.c

@@ -19,12 +19,12 @@ parselist(const char *s)
 	for (i = 0; (p = strsep(&tmp, " ,")); i++) {
 		if (*p == '\0')
 			eprintf("empty field in tablist\n");
-		tablist = erealloc(tablist, (i + 1) * sizeof(*tablist));
+		tablist = ereallocarray(tablist, i + 1, sizeof(*tablist));
 		tablist[i] = estrtonum(p, 1, MIN(LLONG_MAX, SIZE_MAX));
 		if (i > 0 && tablist[i - 1] >= tablist[i])
 			eprintf("tablist must be ascending\n");
 	}
-	tablist = erealloc(tablist, (i + 1) * sizeof(*tablist));
+	tablist = ereallocarray(tablist, i + 1, sizeof(*tablist));
 	return i;
 }