(backport) dynamix/include/SysDevs.php: fix PHP warnings

[desertwitch]

backport of #1854, tests for exec-returned variable/line existence before usage fixes PHP warnings on "System Devices" page encountered under some circumstances:

PHP Warning:  Undefined array key 0 in /usr/local/emhttp/plugins/dynamix/include/SysDevs.php on line 94
PHP Deprecated:  preg_match_all(): Passing null to parameter #2 ($subject) of type string is deprecated in /usr/local/emhttp/plugins/dynamix/include/SysDevs.php on line 94

-- Rysz