Use memmove for overlapping buffer

[rwmjones]

Coverity found the following test where memcpy is used to copy an overlapping buffer. memmove should be used instead.

Error: BUFFER_SIZE (CWE-474):
libmspack-0.10.1alpha/test/md5.c:281: overlapping_buffer: The source buffer "&ctx->buffer[64]" potentially overlaps with the destination buffer "ctx->buffer", which results in undefined behavior for "memcpy". libmspack-0.10.1alpha/test/md5.c:281: remediation: Use memmove instead of "memcpy".
  279|             md5_process_block (ctx->buffer, 64, ctx);
  280|             left_over -= 64;
  281|->           memcpy (ctx->buffer, &ctx->buffer[64], left_over);
  282|           }
  283|         ctx->buflen = left_over;