gif: avoid (wrong) uninitialized warning

My guess is that GCC reports an warning because of the default case.

I don't think we support yet __builtin_unreachable on Linux
Nevertheless it will still be an issue in non-release build

pcsx2/Gif_Unit.h

@@ -79,6 +79,7 @@ struct Gif_Tag {
 		hasAD = false;
 		nRegIdx = 0;
 		isValid = 1;
+		len = 0; // avoid uninitialized compiler warning
 		switch(tag.FLG) {
 			case GIF_FLG_PACKED:
 				nRegs  = ((tag.NREG-1)&0xf) + 1;