libdrgn: fix copying value to big-endian from little-endian

copy_lsbytes() doesn't copy enough bytes when copying from a smaller
little-endian value to a larger big-endian value. This was caught by the
test cases for DW_OP_deref{,_size}, but it can affect other places when
debugging a little-endian target from a big-endian host or vice-versa.

Closes #105.

Signed-off-by: Omar Sandoval <osandov@osandov.com>

libdrgn/serialize.h

@@ -65,7 +65,7 @@ static inline void copy_lsbytes(void *dst, size_t dst_size,
 	} else {
 		memset(d, 0, dst_size - size);
 		if (src_little_endian) {
-			for (size_t i = dst_size - size; i < size; i++)
+			for (size_t i = dst_size - size; i < dst_size; i++)
 				d[i] = s[dst_size - 1 - i];
 		} else {
 			memcpy(d + dst_size - size, s + src_size - size, size);