fix undefined behavior in ptrace

src/linux/ptrace.c

@@ -7,14 +7,18 @@ long ptrace(int req, ...)
 {
 	va_list ap;
 	pid_t pid;
-	void *addr, *data, *addr2;
+	void *addr, *data, *addr2 = 0;
 	long ret, result;
 
 	va_start(ap, req);
 	pid = va_arg(ap, pid_t);
 	addr = va_arg(ap, void *);
 	data = va_arg(ap, void *);
-	addr2 = va_arg(ap, void *);
+	/* PTRACE_{READ,WRITE}{DATA,TEXT} (16...19) are specific to SPARC. */
+#ifdef PTRACE_READDATA
+	if ((unsigned)req - PTRACE_READDATA < 4)
+		addr2 = va_arg(ap, void *);
+#endif
 	va_end(ap);
 
 	if (req-1U < 3) data = &result;