media: cec: stm32: prevent out-of-bounds write on RX overflow

In the Linux kernel, the following vulnerability has been resolved: media: cec: stm32: prevent out-of-bounds write on RX overflow stm32_rx_done() appends each received CEC byte to rx_msg.msg[] using rx_msg.len as the write index, incrementing it on every RXBR (receive-byte-ready) interrupt without checking it against the buffer size: cec->rx_msg.msg[cec->rx_msg.len++] = val & 0xFF; rx_msg.msg[] is a fixed CEC_MAX_MSG_SIZE (16) byte array in struct cec_msg, and rx_msg.len is only reset on RXACKE/RXOVR or after a completed message (RXEND). The number of bytes received before RXEND is decided by the remote CEC device (it sets EOM), not by the driver. A peer that keeps sending bytes without ending the message drives RXBR repeatedly, pushing rx_msg.len past 16 and writing peer-controlled bytes out of bounds into the surrounding memory. This is reachable in normal operation once the driver has probed and receiving is enabled, from the IRQ thread, without any local privilege. The length check in the CEC core runs on the consumer side, after the byte has been stored, so it does not prevent the overflow. Bound the index in the driver before the store, as the other platform CEC drivers already do (e.g. tegra_cec), dropping the excess bytes of an overlong frame. Found by static analysis tool CodeQL.