tci: don't write zero for reloc in tci_out_label

If tci_out_label is called in the context of tcg_gen_code_search_pc, we
could be overwriting an already patched relocation with zero -- and not
repatch it because the set_label is past search_pc, causing a QEMU crash
when it tries to branch to a zero label.

Not writing anything to the relocation area seems to be in line with what
other backends do from the couple I looked at (x86, ppc).

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
