expr: tonum: handle case where result was previously calculated

As pointed out in a mail to dev expr was segfaulting when multiple
math operations were specified on the command line: eg. 'expr 3 \*
2 + 1'. This happens because the tonum(), introduced in e50d533,
assumed that v->str was always non null. parse() guarantees this
for user input but this is not the case when doop() is called with
the result of a previous calculation. However in that case we know
that v->num is already valid so we can simply return.

expr.c

@@ -21,7 +21,13 @@ static void
 tonum(struct val *v)
 {
 	const char *errstr;
-	long long d = strtonum(v->str, LLONG_MIN, LLONG_MAX, &errstr);
+	long long d;
+
+	/* check if val is the result of an earlier calculation */
+	if (!v->str)
+		return;
+
+	d = strtonum(v->str, LLONG_MIN, LLONG_MAX, &errstr);
 	if (errstr)
 		enprintf(2, "error: expected integer, got %s\n", v->str);
 	v->num = d;