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

[sivachandra]

Summary

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

[sivachandra]

I am going to land this so that the riscv builder can turn green. Please do not hesitate to review post commit.

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

int Thread::get_name(cpp::StringStream &name) const {
   char name_buffer[NAME_SIZE_MAX];
 
   if (*this == self) {
     // If we are getting the name of the current thread, then we can
     // use the syscall to get the name.
     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;
   }
 
   char path_name_buffer[THREAD_NAME_PATH_SIZE];
   cpp::StringStream path_stream(path_name_buffer);
   construct_thread_name_file_path(path_stream, attrib->tid);
 #ifdef SYS_open
   int fd = __llvm_libc::syscall_impl(SYS_open, path_name_buffer, O_RDONLY);