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

strings.c

@@ -11,11 +11,11 @@ static char *format = "";
 static void
 strings(FILE *fp, const char *fname, size_t len)
 {
-	Rune r, *rbuf;
+	Rune r, *rbuf = NULL;
 	size_t i, bread;
 	off_t off;
 
-	rbuf = emalloc(len * sizeof(*rbuf));
+	rbuf = ereallocarray(rbuf, len, sizeof(*rbuf));
 
 	for (off = 0, i = 0; (bread = efgetrune(&r, fp, fname)); ) {
 		off += bread;