Fix instances of -Wnull-pointer-arithmetic

As of https://reviews.llvm.org/D98798, clang warns on UB from
subtraction against a null pointer. This addresses one of the new
instances of this warning.

Bug: 75276
Change-Id: Iebd43796deae1dccc18ababcff1e75769b518cfb
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/libpng/+/529201
Reviewed-by: Shai Barack <shayba@google.com>
2 files changed