Fix -Wuninitialized-const-pointer for dummy buffer

[etan-status]

Compiling on macOS produces a -Wuninitialized-const-pointer warning. Initialize to 0 to avoid it.

./snappy-test.cc:469:52: error: variable 'dummyin' is uninitialized when passed as a const pointer argument here [-Werror,-Wuninitialized-const-pointer]
  469 |   if ( UncompressChunkOrAll(&dummyout, &dummylen, &dummyin, 0, Z_FINISH)
      |                                                    ^~~~~~~
1 error generated.

% sw_vers
ProductName:		macOS
ProductVersion:		26.5.2
BuildVersion:		25F84

% clang --version
Apple clang version 21.0.0 (clang-2100.1.1.101)
Target: arm64-apple-darwin25.5.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

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[etan-status]

It's a BSD project...

[etan-status]

Also fixes the equivalent -Wmaybe-uninitialized on GCC:

./snappy-test.cc:469:28: warning: ‘dummyin’ may be used uninitialized [-Wmaybe-uninitialized]
  469 |   if ( UncompressChunkOrAll(&dummyout, &dummylen, &dummyin, 0, Z_FINISH)
      |        ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
./snappy-test.cc:443:5: note: by argument 4 of type ‘const Bytef*’ {aka ‘const unsigned char*’} to ‘int snappy::ZLib::UncompressChunkOrAll(Bytef*, uLongf*, const Bytef*, uLong, int)’ declared here
  443 | int ZLib::UncompressChunkOrAll(Bytef *dest, uLongf *destLen,
      |     ^~~~
./snappy-test.cc:467:9: note: ‘dummyin’ declared here
  467 |   Bytef dummyin, dummyout;
      |         ^~~~~~~