[libfuzzer] using png_sig_cmp correctly
[libfuzzer] using png_sig_cmp correctly

According to doc:

Description

png_sig_cmp() checks whether the given number of bytes match the PNG
signature starting from the start position. The function shall return
non-zero if num_to_check == 0 or start > 7.

Return Value

Zero - the given number of bytes starting from start position match the
respective bytes of the PNG signature. Non-zero - the given number of
bytes starting from start position do not match the respective bytes of
the PNG signature or num_to_check == 0 or start > 7.

BUG=

Review URL: https://codereview.chromium.org/1815703002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#382032}

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