Fix fread return value check

nilmdb/server/rocket.c

@@ -537,7 +537,7 @@ static PyObject *Rocket_extract_string(Rocket *self, PyObject *args)
 			/* read and format in a loop */			\
 			for (i = 0; i < self->layout_count; i++) {	\
 				if (fread(&disktype, bytes,		\
-					  1, self->file) < 0)		\
+					  1, self->file) != 1)		\
 					goto err;			\
 				disktype = letoh(disktype);		\
 				ret = sprintf(&str[len], " " fmt,	\