trace_events_hist: add check for return value of 'create_hist_field'

In the Linux kernel, the following vulnerability has been resolved:

trace_events_hist: add check for return value of 'create_hist_field'

Function 'create_hist_field' is called recursively at

trace_events_hist.c:1954 and can return NULL-value that's why we have

to check it to avoid null pointer dereference.

Found by Linux Verification Center (linuxtesting.org) with SVACE.