问为何华润报告“2码无效阅读”？

EN

struct item
{
    int a;
};
int main()
{
    item *a = (item *)malloc(sizeof(item));
    item *b = (item *)malloc(sizeof(item));
    short *c = (short *)b;
    c += 3; 
    memcpy(a, c, sizeof(int));
    free(a);
    free(b);
    return 0;
}

==19134== Invalid read of size 2
==19134== at 0x4C2F7E0: memcpy@@GLIBC_2.14 (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==19134== by 0x400625: main (main.cpp:19)
==19134== Address 0x51fd096 is 2 bytes after a block of size 4 alloc'd
==19134== at 0x4C2AB80: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==19134== by 0x4005FC: main (main.cpp:16)