Do not reset get_regs_error unnecessarily

As get_regs() is now guaranteed to set get_regs_error,
there is no need to reset get_regs_error before get_regs() call.

* strace.c (trace): Call clear_regs() iff !WIFSTOPPED.
diff --git a/strace.c b/strace.c
index 250f49c..f851c98 100644
--- a/strace.c
+++ b/strace.c
@@ -2311,9 +2311,10 @@
 			return true;
 	}
 
-	clear_regs();
 	if (WIFSTOPPED(status))
 		get_regs(pid);
+	else
+		clear_regs();
 
 	event = (unsigned int) status >> 16;