Upstart/Init hangs or crashes during the boot

Issue

• Server hangs indefinitely during the boot with no error message.
• Server panics during the boot with following message:

init: environ.c:470: Assertion failed in environ_expand: string != NULL
init: Caught abort, core dumped
Kernel panic - not syncing: Attempted to kill init!
Pid: 1, comm: init Not tainted 2.6.32-431.20.3.el6.x86_64 #1
Call Trace:
 [<ffffffff8152812c>] ? panic+0xa7/0x16f
 [<ffffffff81332036>] ? get_current_tty+0x66/0x70
 [<ffffffff81077332>] ? do_exit+0x862/0x870
 [<ffffffff81077398>] ? do_group_exit+0x58/0xd0
 [<ffffffff81077427>] ? sys_exit_group+0x17/0x20
 [<ffffffff8100b072>] ? system_call_fastpath+0x16/0x1b