Memory leak in retrieve_sensors()

[dgraziotin]

As reported by valgrind

==30820== 
==30820== HEAP SUMMARY:
==30820==     in use at exit: 51 bytes in 1 blocks
==30820==   total heap usage: 305 allocs, 304 frees, 525,754 bytes allocated
==30820== 
==30820== 51 bytes in 1 blocks are definitely lost in loss record 1 of 1
==30820==    at 0x4C2CB3F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==30820==    by 0x10B17C: retrieve_sensors (mbpfan.c:150)
==30820==    by 0x10BD47: mbpfan (mbpfan.c:513)
==30820==    by 0x10ADEC: go_daemon (daemon.c:225)
==30820==    by 0x10C478: main (main.c:148)
==30820== 
==30820== LEAK SUMMARY:
==30820==    definitely lost: 51 bytes in 1 blocks
==30820==    indirectly lost: 0 bytes in 0 blocks
==30820==      possibly lost: 0 bytes in 0 blocks
==30820==    still reachable: 0 bytes in 0 blocks
==30820==         suppressed: 0 bytes in 0 blocks
==30820== 
==30820== For counts of detected and suppressed errors, rerun with: -v
==30820== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)

path_begin needs to be free'd up