segfault on context switch when ULT is enable

[ael-code]

when User Level Thread is enabled the execution goes in segmentation fault.

On commit 1bb46a3 at origin/develop.

Whit the following config:

   * ROOT-Sim Configuration on Current Machine *

CPU Architecture.......... : x86_64 (available supports: rdtcs SSE3 SSSE3 SSE4.2 )
Operating System.......... : linux-gnu
Debugging Support......... : Enabled
User-Level Threads........ : Enabled (use --disable-ult if not wanted)
Parallel Allocator........ : Enabled (By default, use --disable-allocator if not wanted)
NUMA Subsystem............ : Disabled
LP Preemption Support..... : Disabled (libtimestretch not found)
LP Rebinding.............. : Enabled (By default, use --disable-rebinding if not wanted)

The line that trigger the segfault is the following
https://github.com/HPDCS/ROOT-Sim/blob/develop/src/scheduler/scheduler.c#L391