Relax pre-getFrameCount frameInfo assertion in Codec_frames

Codecs that parse frame 0 during header decoding (such as
SkPngRustCodec) legitimately have frame 0 available upon creation.
If getFrameInfo(0) returns true, validate that frame 0 is an
independent frame rather than requiring false.

Change-Id: Ic445e0c641827fac5687128b232ade357da4caaa
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1367618
Reviewed-by: Łukasz Anforowicz <lukasza@google.com>
Commit-Queue: Elliot Sisteron <elliotsisteron@google.com>