Rework utf16-to-ascii to avoid snprintf

There isn't a standard snprintf in MS toolchains until VS 2015,
so use C++ iostreams stuff instead.

Author: smowton

src/util/unicode.cpp

@@ -9,6 +9,8 @@ Author: Daniel Kroening, kroening@kroening.com
 #include <cstring>
 #include <locale>
 #include <codecvt>
+#include <sstream>
+#include <iomanip>
 
 #include "unicode.h"
 
@@ -277,19 +279,20 @@ std::wstring utf8_to_utf16_little_endian(const std::string& in)
 
 std::string utf16_little_endian_to_ascii(const std::wstring& in)
 {
-  std::string result;
+  std::ostringstream result;
   std::locale loc;
   for(const auto c : in)
   {
     if(c<=255 && isprint(c, loc))
-      result+=(unsigned char)c;
+      result << (unsigned char)c;
     else
     {
-      result+="\\u";
-      char hex[5];
-      snprintf(hex, sizeof(hex), "%04x", (wchar_t)c);
-      result+=hex;
+      result << "\\u"
+             << std::hex
+             << std::setw(4)
+             << std::setfill('0')
+             << (unsigned int)c;
     }
   }
-  return result;
+  return result.str();
 }