leveldb: Check slice length in Footer::DecodeFrom()

Without this check decoding the footer in Table::Open() can read
uninitialized bytes from a buffer allocated on the stack if the file
was unexpectedly short.

In practice this is probably fine since this function validates a magic
number but MSan complains about branching on uninitialized data.

PiperOrigin-RevId: 525271012

table/format.cc

@@ -41,6 +41,10 @@ void Footer::EncodeTo(std::string* dst) const {
 }
 
 Status Footer::DecodeFrom(Slice* input) {
+  if (input->size() < kEncodedLength) {
+    return Status::Corruption("not an sstable (footer too short)");
+  }
+
   const char* magic_ptr = input->data() + kEncodedLength - 8;
   const uint32_t magic_lo = DecodeFixed32(magic_ptr);
   const uint32_t magic_hi = DecodeFixed32(magic_ptr + 4);