[MSan] Fix check overflow in a test case

Previously, msan_check_mem_is_initialized.cpp initialized a stack array
with size of 32-byte, but check the shaodw for offset range [12, 42),
exceeding the stack array size.

MSan does not guarantee that the shadow corresponding to the overflow
part  is 0, so it is undefined to require the overflow part to be
unpoisoned

Author: Camsyn

compiler-rt/test/msan/msan_check_mem_is_initialized.cpp

@@ -12,7 +12,7 @@ int main(void) {
   __msan_poison(p + 10, 2);
 
   __msan_check_mem_is_initialized(p, 10);
-  __msan_check_mem_is_initialized(p + 12, 30);
+  __msan_check_mem_is_initialized(p + 12, 20);
 #ifdef POSITIVE
   __msan_check_mem_is_initialized(p + 5, 20);
   // CHECK: Uninitialized bytes in __msan_check_mem_is_initialized at offset 5 inside [0x{{.*}}, 20)