Fix a race condition in the "arenas.extend" mallctl.

Fix a race condition in the "arenas.extend" mallctl that could lead to
internal data structure corruption.  The race could be hit if one
thread called the "arenas.extend" mallctl while another thread
concurrently triggered initialization of one of the lazily created
arenas.

ChangeLog

@@ -9,6 +9,8 @@ found in the git revision history:
 * 3.4.1 (XXX)
 
   Bug fixes:
+  - Fix a race in the "arenas.extend" mallctl that could cause memory corruption
+    of internal data structures and subsequent crashes.
   - Fix Valgrind integration flaws that caused Valgrind warnings about reads of
     uninitialized memory in:
     + arena chunk headers