Fix segmentation fault in strhandle()

We cannot pass strescseq.args[0] to atoi when nargs is zero,
because in this case it will be null.

st.c

@@ -2268,8 +2268,7 @@ strhandle(void) {
 
 	term.esc &= ~(ESC_STR_END|ESC_STR);
 	strparse();
-	narg = strescseq.narg;
-	par = atoi(strescseq.args[0]);
+	par = (narg = strescseq.narg) ? atoi(strescseq.args[0]) : 0;
 
 	switch(strescseq.type) {
 	case ']': /* OSC -- Operating System Command */