bvar: fix bad_weak_ptr crash in AgentCombiner::get_or_create_tls_agent

[Gmymymy]

Problem

When a bvar object (Reducer, IntRecorder, Percentile) is destroyed
while another thread is concurrently writing to it via operator<<, the
following crash occurs:

terminate called after throwing an instance of 'std::bad_weak_ptr'
  what():  bad_weak_ptr
Aborted (core dumped)

This has been observed in high-concurrency RDMA performance testing
(see #3288).

Root Cause

AgentCombiner inherits from std::enable_shared_from_this.
In get_or_create_tls_agent(), when an agent's combiner weak_ptr has
expired (the previous combiner was destroyed and the slot was reused), the
method calls this->shared_from_this() to re-bind the agent to the
current combiner.

shared_from_this() throws std::bad_weak_ptr if the object is not
currently managed by any shared_ptr. This can happen in a race where:

1. The AgentCombiner is the last object keeping a bvar alive
2. Another thread releases the last shared_ptr to it (e.g., the owning
   Reducer goes out of scope or is destroyed during program shutdown)
3. A third thread is simultaneously inside get_or_create_tls_agent(),
   past the agent->combiner.expired() check, and calls shared_from_this()
   on the now-unmanaged object → bad_weak_ptr → terminate()

Fix

Wrap shared_from_this() in a try/catch(std::bad_weak_ptr). When caught,
return NULL and silently skip the recording. This is safe: the metric is
being torn down, so dropping a write in flight is acceptable and far
preferable to crashing.

Also remove the LOG(FATAL) in the three operator<< callers that fire
when get_or_create_tls_agent() returns NULL:

• For allocation failure, get_or_create_tls_agent() already calls
  LOG(FATAL) internally (which aborts); the outer LOG(FATAL) was
  unreachable in that case.
• For the new combiner-expired path, calling LOG(FATAL) would
  incorrectly abort the process for a benign race during teardown.

Affected files

File	Change
src/bvar/detail/combiner.h	catch bad_weak_ptr in get_or_create_tls_agent()
src/bvar/reducer.h	remove outer LOG(FATAL) from operator<<
src/bvar/recorder.h	remove outer LOG(FATAL) from operator<<
src/bvar/detail/percentile.cpp	remove outer LOG(FATAL) from operator<<

Fixes #3288

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