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

paste.c

@@ -85,8 +85,8 @@ usage(void)
 int
 main(int argc, char *argv[])
 {
-	struct fdescr *dsc;
-	Rune  *delim;
+	struct fdescr *dsc = NULL;
+	Rune  *delim = NULL;
 	size_t i, len;
 	int    seq = 0;
 	char  *adelim = "\t";
@@ -107,11 +107,11 @@ main(int argc, char *argv[])
 
 	/* populate delimiters */
 	unescape(adelim);
-	delim = emalloc((utflen(adelim) + 1) * sizeof(*delim));
+	delim = ereallocarray(delim, utflen(adelim) + 1, sizeof(*delim));
 	len = utftorunestr(adelim, delim);
 
 	/* populate file list */
-	dsc = emalloc(argc * sizeof(*dsc));
+	dsc = ereallocarray(dsc, argc, sizeof(*dsc));
 
 	for (i = 0; i < argc; i++) {
 		if (strcmp(argv[i], "-") == 0)