tracing: Fix potential double free in create_var_ref()

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

tracing: Fix potential double free in create_var_ref()

In create_var_ref(), init_var_ref() is called to initialize the fields

of variable ref_field, which is allocated in the previous function call

to create_hist_field(). Function init_var_ref() allocates the

corresponding fields such as ref_field->system, but frees these fields

when the function encounters an error. The caller later calls

destroy_hist_field() to conduct error handling, which frees the fields

and the variable itself. This results in double free of the fields which

are already freed in the previous function.

Fix this by storing NULL to the corresponding fields when they are freed

in init_var_ref().