files: check the result of fdopen(), to avoid a possible crash

When safe_tempfile() returns a valid filename, it should also
return a valid open stream.

This fixes https://savannah.gnu.org/bugs/?61064.

Bug existed since version 1.3.8, commit 5e068c60.

src/files.c

@@ -1469,13 +1469,15 @@ char *safe_tempfile(FILE **stream)
 
 	fd = mkstemp(tempfile_name);
 
-	if (fd == -1) {
+	*stream = (fd > 0) ? fdopen(fd, "r+b") : NULL;
+
+	if (*stream == NULL) {
+		if (fd > 0)
+			close(fd);
 		free(tempfile_name);
 		return NULL;
 	}
 
-	*stream = fdopen(fd, "r+b");
-
 	return tempfile_name;
 }