Description: serial: 8250: fix null-ptr-deref in serial8250_start_tx()
References:
 https://lkml.org/lkml/2020/7/21/80
Notes:
Bugs:
upstream: released (5.8-rc7) [f4c23a140d80ef5e6d3d1f8f57007649014b60fa]
4.19-upstream-stable: released (4.19.135) [c358255ff1dfa51ddbcbc8dfcc4eaa5719008daa]
4.9-upstream-stable: released (4.9.232) [81e57039ebbc2c0867f2c04bb3cb205b4edd0141]
sid: released (5.7.17-1)
4.19-buster-security: released (4.19.146-1)
4.9-stretch-security: released (4.9.240-1)