Fix interpolation/STRFMT overflow issues (#838)

Widths and fractional widths greater than 255 would overflow a
uint8_t and wrap around to smaller values.

Total formatted lengths greater than the avilable buffer size
would overflow it and potentially corrupt memory.

Fixes #830
Closes #831

include/asm/format.h

@@ -28,9 +28,9 @@ struct FormatSpec {
 	bool prefix;
 	bool alignLeft;
 	bool padZero;
-	uint8_t width;
+	size_t width;
 	bool hasFrac;
-	uint8_t fracWidth;
+	size_t fracWidth;
 	int type;
 	bool valid;
 };