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

tr.c

@@ -71,16 +71,16 @@ rstrmatch(Rune *r, char *s, size_t n)
 static size_t
 makeset(char *str, struct range **set, int (**check)(Rune))
 {
-	Rune  *rstr;
+	Rune  *rstr = NULL;
 	size_t len, i, j, m, n;
 	size_t q, setranges = 0;
 	int    factor, base;
 
 	/* rstr defines at most len ranges */
 	unescape(str);
-	rstr = emalloc((utflen(str) + 1) * sizeof(*rstr));
+	rstr = ereallocarray(rstr, utflen(str) + 1, sizeof(*rstr));
 	len = utftorunestr(str, rstr);
-	*set = emalloc(len * sizeof(**set));
+	*set = ereallocarray(*set, len, sizeof(**set));
 
 	for (i = 0; i < len; i++) {
 		if (rstr[i] == '[') {