shared: Fix use after free in read_watch_destroy

shared: Fix use after free in read_watch_destroy

read_watch_destroy is called when received_data returns FALSE.
free mgmt in read_watch_destroy instead of received_data to avoid
use after free.

Invalid write of size 4
   at 0x8051604: read_watch_destroy (mgmt.c:271)
   by 0x48C7468E: g_source_callback_unref (gmain.c:1457)
   by 0x48C77287: g_main_context_dispatch (gmain.c:2723)
   by 0x48C774FF: g_main_context_iterate.isra.22 (gmain.c:3290)
   by 0x48C77962: g_main_loop_run (gmain.c:3484)
   by 0x805393E: tester_run (tester.c:784)
   by 0x804D1C7: main (mgmt-tester.c:2558)
 Address 0x4039b80 is 16 bytes inside a block of size 76 free'd
   at 0x4007F0F: free (vg_replace_malloc.c:446)
   by 0x48C7D44B: standard_free (gmem.c:98)
   by 0x48C7D607: g_free (gmem.c:252)
   by 0x8051BB6: received_data (mgmt.c:337)
   by 0x48CBA72E: g_io_unix_dispatch (giounix.c:167)
   by 0x48C7715A: g_main_context_dispatch (gmain.c:2715)
   by 0x48C774FF: g_main_context_iterate.isra.22 (gmain.c:3290)
   by 0x48C77962: g_main_loop_run (gmain.c:3484)
   by 0x805393E: tester_run (tester.c:784)
   by 0x804D1C7: main (mgmt-tester.c:2558)