fix(ble_mesh): Reset reassembly buffer at start of each transaction

The reassembly buffer must be reset to its origin at the beginning of every
transaction. prov_msg_recv() pulls the PDU type byte (advancing buf->data by
one) and nothing restores it between transactions. Without this reset,
buf->data drifts forward by one byte per received PDU, causing the segment-0
memcpy to write past the end of the statically allocated rx buffer
(PROV_RX_BUF_SIZE), and the XACT_SEG_DATA() offsets used for continuation
segments to be skewed by the accumulated drift.

components/bt/esp_ble_mesh/core/prov_common.c

@@ -199,6 +199,17 @@ bool bt_mesh_gen_prov_start(struct bt_mesh_prov_link *link,
         return false;
     }
 
+    /* Reset the reassembly buffer so every transaction starts from the
+     * buffer origin. prov_msg_recv() pulls the PDU type byte (advancing
+     * buf->data by one) and nothing restores it between transactions, so
+     * without this reset buf->data would drift forward by one byte per
+     * received PDU: the segment-0 memcpy below would then write past the
+     * end of the statically allocated rx buffer (PROV_RX_BUF_SIZE), and
+     * the XACT_SEG_DATA() offsets used for continuation segments would
+     * be skewed by the accumulated drift.
+     */
+    net_buf_simple_reset(link->rx.buf);
+
     link->rx.buf->len = net_buf_simple_pull_be16(buf);
     link->rx.id = rx->xact_id;
     link->rx.fcs = net_buf_simple_pull_u8(buf);