Fix crash on font resize resize

if you keep downsizing your fontsize until either xw.ch or xw.cw gets 0,
st crashes, because there is an unchecked division in cresize.

st.c

@@ -2992,7 +2992,7 @@ xloadfonts(char *fontstr, double fontsize) {
 	if(!pattern)
 		die("st: can't open font %s\n", fontstr);
 
-	if(fontsize > 0) {
+	if(fontsize > 1) {
 		FcPatternDel(pattern, FC_PIXEL_SIZE);
 		FcPatternDel(pattern, FC_SIZE);
 		FcPatternAddDouble(pattern, FC_PIXEL_SIZE, (double)fontsize);