fuzz: Fix wallet_bdb_parser 32-bit unhandled fseek error

[maflcko]

std::fseek on 64-bit past the end of the file may work fine (the following read would fail). However, on 32-bit it may fail early.

Fix it, by ignoring the error, treating it similar to a read error.

This was found by OSS-Fuzz.

https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=69414

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Type	Reviewers
ACK	TheCharlatan, brunoerg

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[maflcko]

Example:

$ base64 ~/Downloads/clusterfuzz-testcase-minimized-wallet_bdb_parser-5937261974716416 
ICD//yAgICAAAAAAAAUxYgAAAAkAAAIAAAkgICAgICAAAAAAICAgICAgICAgICAgAAAAIP//ICAg
ICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICD/ICAgICAg////////ICAgICAgICAgICAg
ICAgICAgICAgICAgICAgICAgICAgICAg////ICD//yAgICAgICAgICAgICAgICAgICAg////////
ICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAg////ICAgICAgICAgICAgICAgICAg
ICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgIP///yAgICAg
ICAgICAg////IP//////ICAgICAgICAg//8gICAgICAgICAgICAgICAgICAgICAgICAgICAgICAg
ICAgICAgICAgICAgICAgICAgICD///8g//////8gICAgICAgICD//yAgICAgICAgICAgICAgICAg
ICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAg////ICAgIP//IP//////ICAgICAg
ICAg//8gICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICAgICA=

To test locally:

CONFIG_SITE="$PWD/depends/i686-pc-linux-gnu/share/config.site" ./configure  --enable-fuzz --with-sanitizers=fuzzer && make clean && make  -j $(nproc)
FUZZ=wallet_bdb_parser ./src/test/fuzz/fuzz -runs=1 /tmp/c

Before:

terminate called after throwing an instance of 'std::runtime_error'
  what():  AutoFile::seek: fseek failed: iostream error

After:

(Passes)

[TheCharlatan]

ACK fa7bc9b

[brunoerg]

utACK fa7bc9b