Don't bitshift by negative amounts.

Don't bitshift by negative amounts when encoding/decoding run sizes in
chunk header maps.  This affected systems with page sizes greater than 8
KiB.

Reported by Ingvar Hagelund <ingvar@redpill-linpro.com>.

ChangeLog

@@ -4,6 +4,12 @@ brevity.  Much more detail can be found in the git revision history:
 
     https://github.com/jemalloc/jemalloc
 
+* 4.x.x (XXX)
+
+  Bug fixes:
+  - Don't bitshift by negative amounts when encoding/decoding run sizes in chunk
+    header maps.  This affected systems with page sizes greater than 8 KiB.
+
 * 4.0.0 (August 17, 2015)
 
   This version contains many speed and space optimizations, both minor and