[base] Fix SmallVector's move constructor

The move constructor left the "other" (source) vector in an
unpredictable state, depending on the size: For "big" small-vectors
(using dynamically allocated storage) we would reset it to an empty
vector. "Small" small-vectors on the other hand were not reset.

Fix this to make it possible to reuse a SmallVector after moving its
content to another SmallVector. This also flushes out a bug more easily,
see https://crrev.com/c/4215292.

R=dlehmann@chromium.org
CC=​thibaudm@chromium.org

Change-Id: Ia188c3639e9104dfbeb589bfc49e3228f4cbeda7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/4215297
Reviewed-by: Daniel Lehmann <dlehmann@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#85612}

src/base/small-vector.h

@@ -81,13 +81,13 @@ class SmallVector {
       begin_ = other.begin_;
       end_ = other.end_;
       end_of_storage_ = other.end_of_storage_;
-      other.reset_to_inline_storage();
     } else {
       DCHECK_GE(capacity(), other.size());  // Sanity check.
       size_t other_size = other.size();
       memcpy(begin_, other.begin_, sizeof(T) * other_size);
       end_ = begin_ + other_size;
     }
+    other.reset_to_inline_storage();
     return *this;
   }