pdch_ul_controller: Fix compiler warning on gcc-10.2

pdch_ul_controller.c: In function ‘pdch_ulc_release_tbf’:
pdch_ul_controller.c:217:7: error: ‘item_tbf’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
  217 |    if (item_tbf != tbf)
      |       ^

Change-Id: I42120fdf23753945ebc16bb5469d9fd253c3da37

src/pdch_ul_controller.c

@@ -213,6 +213,8 @@ void pdch_ulc_release_tbf(struct pdch_ulc *ulc, const struct gprs_rlcmac_tbf *tb
 			case PDCH_ULC_NODE_TBF_USF:
 				item_tbf = (struct gprs_rlcmac_tbf *)item->tbf_usf.ul_tbf;
 				break;
+			default:
+				OSMO_ASSERT(0);
 			}
 			if (item_tbf != tbf)
 				continue;