sync drw.{c,h} from dmenu

- drw: minor improvement to the nomatches cache
- overhaul utf8decoding and render invalid utf8 sequences as U+FFFD.

Thanks NRK for these improvements!

ref.
https://git.suckless.org/dwm/commit/8933ebcf50024f4378a78e556b1ac08091197206.html#h0-1

dwm.c

@@ -101,7 +101,6 @@
 #else
 #define ISVISIBLE(C)            ((C->tags & C->mon->tagset[C->mon->seltags]))
 #endif // ATTACHASIDE_PATCH
-#define LENGTH(X)               (sizeof X / sizeof X[0])
 #define MOUSEMASK               (BUTTONMASK|PointerMotionMask)
 #define WIDTH(X)                ((X)->w + 2 * (X)->bw)
 #define HEIGHT(X)               ((X)->h + 2 * (X)->bw)