Don't strip leading 0s in stdnse.tobinary

Every place this function is used, the result is padded with 0s anyway,
so may as well not strip them here. Didn't remove the padding code since
this could return only 4 chars, and most padding is to 8-char width.

nselib/formulas.lua

@@ -70,14 +70,16 @@ local function chi2(data, num_cats)
     return x2
 end
 
+local function c_to_bin (c)
+  local n = stdnse.tobinary(c:byte())
+  return ("0"):rep(8-#n)..n
+end
+
 -- Split a string into a sequence of bit strings of the given length.
 -- splitbits("abc", 5) --> {"01100", "00101", "10001", "00110"}
 -- Any short final group is omitted.
 local function splitbits(s, n)
-    local bits = s:gsub(".", function (c)
-        local n = stdnse.tobinary(c:byte())
-        return ("0"):rep(8-#n)..n
-      end)
+    local bits = s:gsub(".", c_to_bin)
 
     local seq = {}
     for i = 1, #bits - n, n do

nselib/stdnse.lua

@@ -260,12 +260,12 @@ do
   };
 
 --- Converts the given number, n, to a string in a binary number format (12
--- becomes "1100").
+-- becomes "1100"). Leading 0s not stripped.
 -- @param n Number to convert.
 -- @return String in binary format.
   function tobinary(n)
     assert(tonumber(n), "number expected");
-    return (("%x"):format(n):gsub("%w", t):gsub("^0*", ""));
+    return (("%x"):format(n):gsub("%w", t))
   end
 end