Switch fakeasm to use snprintf (#663)

tests/fakeasm.t

@@ -4,7 +4,7 @@ local op = setmetatable({},{ __index = function(self,idx)
     return idx
 end })
 C = terralib.includec("stdio.h")
-local function emit(buf,...)
+local function emit(buf,bufsiz,...)
     local str = ""
     local operands = {}
     for i = 1,select("#",...) do
@@ -14,18 +14,18 @@ local function emit(buf,...)
         if e then table.insert(operands,v) end
     end
     str = str.."\n"
-    return `C.sprintf(buf,str,operands)
+    return `C.snprintf(buf,bufsiz,str,operands)
 end
 emit = macro(emit)
 
 local c = "as28"
-terra what(buf : rawstring)
+terra what(buf : rawstring, bufsiz : uint64)
     var b = 3
-    emit(buf,1,3,4,3+3,op.what,b + 3)
+    emit(buf,bufsiz,1,3,4,3+3,op.what,b + 3)
 end
 
 local buf = terralib.new(int8[128])
-what(buf)
+what(buf,128)
 local ffi = require("ffi")
 local s = ffi.string(buf)
 assert("1 3 4 6 what 6 \n" == s)