Check if file is actually a file object (#32)

If it is not a file like object, the function would return -1. This
would eventually lead to a segmention fault later on.

yara-python.c

@@ -1829,10 +1829,17 @@ static PyObject* yara_compile(
     else if (file != NULL)
     {
       fd = dup(PyObject_AsFileDescriptor(file));
+      if (fd != -1) {
         fh = fdopen(fd, "r");
         error = yr_compiler_add_file(compiler, fh, NULL, NULL);
         fclose(fh);
       }
+      else {
+        result = PyErr_Format(
+            PyExc_TypeError,
+            "'file' is not a file object");
+      }
+    }
     else if (sources_dict != NULL)
     {
       if (PyDict_Check(sources_dict))