Fix JBMC crash on windows in NondetEnumOpaqueReturn test

String literals with multibyte utf-8 characters in java cause `ch` to be
negative. Passing a negative value into `std::isalnum` results in
undefined behaviour. On windows the observed behaviour is a popup error
message about a failed assertion in the run time library. This stalls
the CI process as none of the buttons on the popup message ever get
pressed. Escaping these characters is sufficient to fix the stalling
test.

Author: thomasspriggs

src/util/string_utils.cpp

@@ -9,6 +9,7 @@ Author: Daniel Poetzl
 #include "string_utils.h"
 #include "exception_utils.h"
 #include "invariant.h"
+#include "narrow.h"
 
 #include <algorithm>
 #include <cassert>
@@ -156,9 +157,10 @@ std::string escape_non_alnum(const std::string &to_escape)
   std::ostringstream escaped;
   for(auto &ch : to_escape)
   {
+    const auto uch = narrow<int>(narrow_cast<unsigned char>(ch));
     if(ch == '_')
       escaped << "__";
-    else if(isalnum(ch))
+    else if(isalnum(uch))
       escaped << ch;
     else
       escaped << '_' << std::hex << std::setfill('0') << std::setw(2)