DecodeUTF8: use unsigned operand

gframe/bufferio.h

@@ -98,16 +98,16 @@ public:
 		const char* p = src;
 		wchar_t* wp = wstr;
 		while(*p != 0) {
-			if((*p & 0x80) == 0) {
+			if(((unsigned)*p & 0x80) == 0) {
 				*wp = *p;
 				p++;
-			} else if((*p & 0xe0) == 0xc0) {
+			} else if(((unsigned)*p & 0xe0) == 0xc0) {
 				*wp = (((unsigned)p[0] & 0x1f) << 6) | ((unsigned)p[1] & 0x3f);
 				p += 2;
-			} else if((*p & 0xf0) == 0xe0) {
+			} else if(((unsigned)*p & 0xf0) == 0xe0) {
 				*wp = (((unsigned)p[0] & 0xf) << 12) | (((unsigned)p[1] & 0x3f) << 6) | ((unsigned)p[2] & 0x3f);
 				p += 3;
-			} else if((*p & 0xf8) == 0xf0) {
+			} else if(((unsigned)*p & 0xf8) == 0xf0) {
 				if (sizeof(wchar_t) == 2) {
 					unsigned unicode = (((unsigned)p[0] & 0x7) << 18) | (((unsigned)p[1] & 0x3f) << 12) | (((unsigned)p[2] & 0x3f) << 6) | ((unsigned)p[3] & 0x3f);
 					unicode -= 0x10000;