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

sed.c

@@ -275,7 +275,7 @@ resize(void **ptr, size_t *nmemb, size_t size, size_t new_nmemb, void **next)
 	void *n, *tmp;
 
 	if (new_nmemb) {
-		tmp = erealloc(*ptr, new_nmemb * size);
+		tmp = ereallocarray(*ptr, new_nmemb, size);
 	} else { /* turns out realloc(*ptr, 0) != free(*ptr) */
 		free(*ptr);
 		tmp = NULL;
@@ -596,7 +596,10 @@ chompr(char *s, Rune rune)
 Rune *
 strtorunes(char *s, size_t nrunes)
 {
-	Rune *rs = emalloc(sizeof(*rs) * nrunes + 1), *rp = rs;
+	Rune *rs = NULL, *rp;
+
+	rs = ereallocarray(rs, nrunes + 1, sizeof(*rs));
+	rp = rs;
 
 	while (nrunes--)
 		s += chartorune(rp++, s);