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

test/asm/interpolation-overflow.asm

@@ -0,0 +1,4 @@
+; It seems that \1 was the easiest way to notice the memory corruption that
+; resulted from this overflow
+x = 0
+{.99999999f:x}\1