util: Fix error path for virPCIGetVirtualFunctions

util: Fix error path for virPCIGetVirtualFunctions

If we get to the error: label and clear out the *virtual_functions[]
pointers and then return w/ error to the caller - the caller has it's
own cleanup of the same array in the out: label which is keyed off the
value of num_virt_fns, which wasn't reset to 0 in the called function
leading to a possible problem.

Just clear the value (found by Coverity)

Signed-off-by: John Ferlan <jferlan@redhat.com>