Fix handling of invalid smb4.conf auxiliary parameters
I noticed this after upgrading to the latest nightly from a build back in Oct '18, but apparently if there are empty lines (or, at least, lines that do not have a : in them) the smb4_share.conf file will fail to be generated.
This occurs in File "/tmp/mako/usr/local/lib/python3.7/site-packages/middlewared/etc_files/local/smb4_share.conf.py", line 150, in parse_db_config
Since an empty string split on = does not have a second index, this breaks.
One solution would be to check if it actually has two items in the array before trying the update, since terms without an = sign are probably invalid anyways.
This issue is particularly insidious because blank lines are (naturally) allowed in samba.conf (as well as comments) but any user attempting to use them will invisibly cause their configs to fail generation.
Granted it's hidden away under an Advanced toggle, but it is still unexpected behavior.