img_utils: fix static analyzer complaints

Due to https://bugs.llvm.org/show_bug.cgi?id=34365, the static analyzer
complains about use of copied `sp`s. In this case, the copy is entirely
unnecessary, since we're just going to destroy the copied-from sp
anyway.

Speed things up a bit + appease the analyzer by moving instead.

Bug: 27101951
Test: mma. Static analyzer no longer complains.
Change-Id: I08f77c2509dc03a919238b97666be3f458ba7cf3