malloc: Avoid premature fallback to mmap [BZ #20284]

malloc: Avoid premature fallback to mmap [BZ #20284]

Before this change, the while loop in reused_arena which avoids
returning a corrupt arena would never execute its body if the selected
arena were not corrupt.  As a result, result == begin after the loop,
and the function returns NULL, triggering fallback to mmap.

(cherry picked from commit a3b473373ee43a292f5ec68a7fda6b9cfb26a9b0)
(cherry picked from commit a69d26143b80cb1927481509279577c57bc22ba4)