Update lite_unittest to pad varints before calling `internal::VarintParse()`

The `VarintParse()` function is ordinarily called only by the proto parser,
which always provides 16 bytes of "slop" space at the end of the buffer. The
ARM-specific optimized path takes advantage of this by always reading at least
8 bytes. However, this caused some test failures in msan due to unit tests not
providing a sufficient amount of initialized padding. This CL fixes the problem
by making sure the unit tests initialize the full 10-byte buffer and adding a
comment stating that this is a precondition of the function.

PiperOrigin-RevId: 592366291

src/google/protobuf/lite_unittest.cc

@@ -8,6 +8,7 @@
 // Author: kenton@google.com (Kenton Varda)
 
 #include <climits>
+#include <cstdint>
 #include <iostream>
 #include <limits>
 #include <string>
@@ -104,7 +105,7 @@ void SetSomeTypesInEmptyMessageUnknownFields(
 
 TEST(ParseVarintTest, Varint32) {
   auto test_value = [](uint32_t value, int varint_length) {
-    uint8_t buffer[10];
+    uint8_t buffer[10] = {0};
     uint8_t* p = io::CodedOutputStream::WriteVarint32ToArray(value, buffer);
     ASSERT_EQ(p - buffer, varint_length) << "Value = " << value;
 
@@ -131,7 +132,7 @@ TEST(ParseVarintTest, Varint32) {
 
 TEST(ParseVarintTest, Varint64) {
   auto test_value = [](uint64_t value, int varint_length) {
-    uint8_t buffer[10];
+    uint8_t buffer[10] = {0};
     uint8_t* p = io::CodedOutputStream::WriteVarint64ToArray(value, buffer);
     ASSERT_EQ(p - buffer, varint_length) << "Value = " << value;
 

src/google/protobuf/parse_context.h

@@ -847,6 +847,7 @@ static const char* VarintParseSlowArm(const char* p, uint64_t* out,
 }
 #endif
 
+// The caller must ensure that p points to at least 10 valid bytes.
 template <typename T>
 PROTOBUF_NODISCARD const char* VarintParse(const char* p, T* out) {
 #if defined(__aarch64__) && defined(ABSL_IS_LITTLE_ENDIAN)