Fix sigreturn decoding on MIPS

The "return 0" line was accidentally deleted circa 2007,
which made sigreturn on MIPS always display "= 0" return
instead of more informative " = ? (mask now [MASK])".

* strace.c (sys_sigreturn): Add wrongly deleted "return 0" line

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
diff --git a/signal.c b/signal.c
index f2d7277..d2e5953 100644
--- a/signal.c
+++ b/signal.c
@@ -1477,6 +1477,7 @@
 		tcp->u_arg[0] = 0;
 		sp = regs.regs[29];
 		if (umove(tcp, sp, &si) < 0)
+			return 0;
 		tcp->u_arg[0] = 1;
 		tcp->u_arg[1] = si.si_mask;
 	} else {