load: Change buffer_pos to be a size_t

buffer_pos should be type size_t, because it's used to store the
current position in the read buffer. Also, it never can be negative.

src/load.c

@@ -48,7 +48,7 @@ typedef struct {
     get_func get;
     void *data;
     char buffer[5];
-    int buffer_pos;
+    size_t buffer_pos;
     int state;
     int line;
     int column, last_column;