[libc] Another fix to add a null-terminator to the pthread name.

This fix is similar to the one done in https://reviews.llvm.org/D148844.
This miss was caught by the risv64 builder.

Differential Revision: https://reviews.llvm.org/D148871

libc/src/__support/threads/linux/thread.cpp

@@ -358,7 +358,7 @@ int Thread::get_name(cpp::StringStream &name) const {
     int retval = __llvm_libc::syscall_impl(SYS_prctl, PR_GET_NAME, name_buffer);
     if (retval < 0)
       return -retval;
-    name << name_buffer;
+    name << name_buffer << cpp::StringStream::ENDS;
     return 0;
   }