[libc] fix msan failure in mempcpy_test (#75532)

Internal builds of the unittests with msan flagged mempcpy_test.

    ==6862==WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0x55e34d7d734a in length
llvm-project/libc/src/__support/CPP/string_view.h:41:11
#1 0x55e34d7d734a in string_view
llvm-project/libc/src/__support/CPP/string_view.h:71:24
#2 0x55e34d7d734a in
__llvm_libc_9999_0_0_git::testing::Test::testStrEq(char const*, char
const*, char const*, char const*,
__llvm_libc_9999_0_0_git::testing::internal::Location)
llvm-project/libc/test/UnitTest/LibcTest.cpp:284:13
#3 0x55e34d7d4e09 in LlvmLibcMempcpyTest_Simple::Run()
llvm-project/libc/test/src/string/mempcpy_test.cpp:20:3
#4 0x55e34d7d6dff in
__llvm_libc_9999_0_0_git::testing::Test::runTests(char const*)
llvm-project/libc/test/UnitTest/LibcTest.cpp:133:8
#5 0x55e34d7d86e0 in main
llvm-project/libc/test/UnitTest/LibcTestMain.cpp:21:10

SUMMARY: MemorySanitizer: use-of-uninitialized-value
llvm-project/libc/src/__support/CP...