Fix CurDir on Windows

* Allow lowercase argument.  (And properly check the sal_Unicode value with
  rtl::isAsciiUpperCase instead of with isalpha, which would cause UB for values
  outside of unsigned char + EOF).

* Use _wgetdcwd to get a UTF-16 path in the first place (instead of erroneously
  converting via createFromAscii and assuming the path only contains 7-bit ASCII
  characters).

* At least with a MSVC 2015 Update 3 --enable-dbgutil build, a call like

    CurDir("A")

  for a non-existent drive A will cause a failure message box

    Microsoft Visual C++ Runtime Library

    Debug Assertion Failed!

    Program: ...\instdir\program\soffice.bin
    File: minkernel\crts\ucrt\src\desktopcrt\misc\getcwd.cpp
    Line: 225

    Expression: ("Invalid Drive", 0)

  though, which appears it can't be intercepted---trying with a
  _set_thread_local_invalid_parameter_handler around the call to _wgetdcwd
  didn't have any effect.

Change-Id: I666f84b0695152c0f2c25de3bae100e58929594a

basic/source/runtime/methods.cxx

@@ -31,6 +31,7 @@
 #include <vcl/msgbox.hxx>
 #include <basic/sbx.hxx>
 #include <svl/zforlist.hxx>
+#include <rtl/character.hxx>
 #include <rtl/math.hxx>
 #include <tools/urlobj.hxx>
 #include <osl/time.h>
@@ -405,24 +406,18 @@ RTLFUNC(CurDir)
             StarBASIC::Error( ERRCODE_BASIC_BAD_ARGUMENT );
             return;
         }
-        else
+        auto c = rtl::toAsciiUpperCase(aDrive[0]);
+        if ( !rtl::isAsciiUpperCase( c ) )
         {
-            nCurDir = (int)aDrive[0];
-            if ( !isalpha( nCurDir ) )
-            {
-                StarBASIC::Error( ERRCODE_BASIC_BAD_ARGUMENT );
-                return;
-            }
-            else
-            {
-                nCurDir -= ( 'A' - 1 );
-            }
+            StarBASIC::Error( ERRCODE_BASIC_BAD_ARGUMENT );
+            return;
         }
+        nCurDir = c - 'A' + 1;
     }
-    char pBuffer[ _MAX_PATH ];
-    if ( _getdcwd( nCurDir, pBuffer, _MAX_PATH ) != nullptr )
+    wchar_t pBuffer[ _MAX_PATH ];
+    if ( _wgetdcwd( nCurDir, pBuffer, _MAX_PATH ) != nullptr )
     {
-        rPar.Get(0)->PutString( OUString::createFromAscii( pBuffer ) );
+        rPar.Get(0)->PutString( OUString( pBuffer ) );
     }
     else
     {