TemporaryBuffer: Clear block pointer list instead of reallocating

TemporaryBuffer: Clear block pointer list instead of reallocating

The block pointer list may have been relatively large, so no need to
make more garbage. Instead, just clear the list and null out all the
elements.

Another possible motivation: a caller may have provided an inaccurate
estimated size, so the list might have been resized several times. If
the list is reused later for a similarly underestimated workload, this
fix will prevent additional resizing on subsequent usages.

Change-Id: I511675035dcff1117381a46c294cc11aded10893