Fix EOLs buffer shortening in wxFFile::ReadAll()

As the comments in the function explain, fread() may return a shorter
buffer than expected due to CRT's implicit conversion of DOS EOLs to
\n.
The logic for handling this was however broken: it NUL-terminated the
buffer appropriately, but that had no effect when later used in
wxString constructor, which used buffer's length for string length.

This resulted in slightly larger strings with uninitialized tails that
were mostly invisible to the user as the tail would disappear anywhere
the string was handled as a NULL-terminated C string. It also caused
occassional UTF-8 conversion failures when the tailing bytes didn't
form a valid sequence.

Fixed by using wxCharBuffer::shrink() to properly truncate the buffer.

src/common/ffile.cpp

@@ -111,7 +111,8 @@ bool wxFFile::ReadAll(wxString *str, const wxMBConv& conv)
         return false;
     }
 
-    buf.data()[length] = 0;
+    // shrink the buffer to possibly shorter data as explained above:
+    buf.shrink(length);
 
     wxString strTmp(buf, conv);
     str->swap(strTmp);