Fix heap profiling crash for realloc(p, 0) case.

Fix prof_realloc() to not call prof_ctx_set() if a sampled object is
being freed via realloc(p, 0).

ChangeLog

@@ -6,6 +6,11 @@ found in the git revision history:
     http://www.canonware.com/cgi-bin/gitweb.cgi?p=jemalloc.git
     git://canonware.com/jemalloc.git
 
+* 3.x.x (XXX not yet released)
+
+  Bug fixes:
+  - Fix heap profiling crash if sampled object is freed via realloc(p, 0).
+
 * 3.0.0 (May 11, 2012)
 
   Although this version adds some major new features, the primary focus is on