Fix a compiler warning.

input.c: In function ‘load_input_file’:
input.c:123:7: warning: ‘fd’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
   len = read(fd, buf->str, BUFSIZE);
       ^
input.c:42:6: note: ‘fd’ was declared here
  int fd;
      ^
diff --git a/input.c b/input.c
index eb73d05..da4e21d 100644
--- a/input.c
+++ b/input.c
@@ -39,7 +39,7 @@
 	struct sr_dev_inst *sdi;
 	GHashTable *mod_args, *mod_opts;
 	GString *buf;
-	int fd;
+	int fd = 0;
 	ssize_t len;
 	char *mod_id;