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/format-truncation.asm

@@ -0,0 +1,15 @@
+num equ 42
+fix equ 123.0
+str equs "hello"
+
+println "{#0260x:num}"
+println "{#-260x:num}"
+println "{0280.260f:fix}"
+println "{260s:str}"
+println "{-260s:str}"
+
+println "<{#0260x:num}>"
+println "<{#-260x:num}>"
+println "<{0280.260f:fix}>"
+println "<{260s:str}>"
+println "<{-260s:str}>"