LU-18836 utils: lctl SIGSEGV in parser.c

LU-18836 utils: lctl SIGSEGV in parser.c

Added additional checks for NULL pointers in execute_line().

Test-Parameters: trivial
Signed-off-by: Frank Sehr <fsehr@whamcloud.com>
Change-Id: I0e470edadd3b4db4c47f0f42f52d34d541290224
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58468
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Cyril Bordage <cbordage@whamcloud.com>
Reviewed-by: Manish Regmi <mregmi@ddn.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>