Fixed endian handling for I;16 getextrema

[radarhere]

Helps #4213

Using the endian handling in

Pillow/src/libImaging/Unpack.c

Lines 1042 to 1046 in b73e3dd

	#ifdef WORDS_BIGENDIAN
	out[0] = tmp[1]; out[1] = tmp[0];
	#else
	memcpy(out, &pixel, sizeof(pixel));
	#endif

as inspiration, fixes getextrema I;16 operations for big-endian.

[hugovk]

Thanks!