extmod/modframebuf: Fix invalid stride for odd widths in GS4_HMSB fmt.

Since the stride is specified in pixels, in a 4-bit horizontal format it
has to always be even, otherwise the computation is wrong and we can
write outside of the buffer sometimes.

extmod/modframebuf.c

@@ -237,12 +237,14 @@ STATIC mp_obj_t framebuf_make_new(const mp_obj_type_t *type, size_t n_args, size
     switch (o->format) {
         case FRAMEBUF_MVLSB:
         case FRAMEBUF_RGB565:
-        case FRAMEBUF_GS4_HMSB:
             break;
         case FRAMEBUF_MHLSB:
         case FRAMEBUF_MHMSB:
             o->stride = (o->stride + 7) & ~7;
             break;
+        case FRAMEBUF_GS4_HMSB:
+            o->stride = (o->stride + 1) & ~1;
+            break;
         default:
             mp_raise_ValueError("invalid format");
     }