Fix small negative UTC-offset output from smb_zonestr()

When passed values between (-1 and -59), this function would output something
nonsensical like: "UTC0:01" or "UTC0:59". Now we'll make sure there's a minus
sign in these conditions (e.g. "UTC-0:01" and "UTC-0:59").

src/smblib/smbstr.c

@@ -241,6 +241,8 @@ char* smb_zonestr(int16_t zone, char* str)
 
 	if(zone>0)
 		plus="+";
+	else if((zone / 60) == 0)
+		plus="-";
 	else
 		plus="";
 	sprintf(str,"UTC%s%d:%02u", plus, zone/60, zone<0 ? (-zone)%60 : zone%60);