Return -1 in io_file_is_empty() if file cannot be accessed.

Ensure files don't appear as empty if fopen() fails (e.g. on temporary
EACCES failures).

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/io.c

@@ -3085,5 +3085,5 @@ io_file_is_empty (char *file)
         }
     }
 
-  return 1;
+  return -1;
 }