don't return unitialised memory on error in ehci_alloc_sqtd()

don't return unitialised memory on error in ehci_alloc_sqtd()

It seems mpi introduced a problem into ehci.c rev 1.162
two weeks ago.  An error check that used to return NULL
now jumps to the end of the function, but sqtd isn't
initialised at that point.

ok miod@ deraadt@