check_snmp: Fix support for inverse thresholds

Commit bd782990566eec91b8312cfc2765a7e2bd9e67da reintroduced support for
inverse threshold ranges such as "2:1", but it broke standard thresholds
such as "1:2" (by converting this range into "1") or "1:" (by converting
this range into "@:2").  This commit fixes those two bugs, plus an
off-by-one error while computing the number of bytes to allocate when
handling inverse thresholds (two additional bytes were allocated where
three were required to hold '@' and ':' and '\0').  While at it, we also
check whether malloc(3) succeeded.

plugins/check_snmp.c

@@ -150,16 +150,18 @@ static char *fix_snmp_range(char *th)
 {
 	double left, right;
 	char *colon, *ret;
-	if (!(colon = strchr(th, ':')))
+
+	if ((colon = strchr(th, ':')) == NULL || *(colon + 1) == '\0')
 		return th;
-	*colon = 0;
 
 	left = strtod(th, NULL);
 	right = strtod(colon + 1, NULL);
-	if (right >= left) {
+	if (right >= left)
 		return th;
-	}
-	ret = malloc(strlen(th) + strlen(colon + 1) + 2);
+
+	if ((ret = malloc(strlen(th) + 2)) == NULL)
+		die(STATE_UNKNOWN, _("Cannot malloc"));
+	*colon = '\0';
 	sprintf(ret, "@%s:%s", colon + 1, th);
 	free(th);
 	return ret;