bluetooth: esp32: make LE discardability length-safe

Determine event discardability after parsing the HCI header and require
at least one byte before reading the LE subevent. This removes a possible
OOB read on malformed/short LE Meta events. Also mark LE Extended
Advertising Report as discardable, matching legacy Advertising Report to
reduce RX pool pressure during heavy scanning.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>