get_regs: guard against subsequent invocations

At this moment there is only one call to get_regs,
but there is going to be more of them, so a guard is needed.

This change also adds a clear_regs call before get_regs,
which essentially reverts commit v4.9-288-ge9bfff6.

* strace.c (trace): Call clear_regs before get_regs.
* syscall.c (get_regs): Skip if get_regs_error != -1.

strace.c

@@ -2295,10 +2295,9 @@ trace(void)
 			return true;
 	}
 
+	clear_regs();
 	if (WIFSTOPPED(status))
 		get_regs(pid);
-	else
-		clear_regs();
 
 	event = (unsigned int) status >> 16;
 

syscall.c

@@ -1156,6 +1156,9 @@ get_regs(pid_t pid)
 #undef USE_GET_SYSCALL_RESULT_REGS
 #ifdef ptrace_getregset_or_getregs
 
+	if (get_regs_error != -1)
+		return;
+
 # ifdef HAVE_GETREGS_OLD
 	/*
 	 * Try PTRACE_GETREGSET/PTRACE_GETREGS first,