Fix pointer wrap around undefined behavior

[sasdf]

zbar/zbar/img_scanner.c

Line 965 in a549566

movedelta(-1, 0);

zbar/zbar/img_scanner.c

Lines 861 to 866 in a549566

	#define movedelta(dx, dy) \
	do { \
	x += (dx); \
	y += (dy); \
	p += (dx) + ((uintptr_t)(dy)*w); \
	} while (0);

This expression movedelta(-1, 0); is expanded to

p += (-1) + ((uintptr_t)(0) * w);

where the RHS is evaluated unsigned as (uintptr_t) -1 i.e. 0xfff...

This pointer addition with unsigned wrap around is an undefined behavior in C.

In the latest clang trunk, the expr is optimized as a constant assignment ( p = -1; ), and segfault in runtime.
https://godbolt.org/z/G8xeMWTo5

This PR fixes the issue by casting the unsigned variable w to a signed type ptrdiff_t.
Given that dx and dy are both int, the RHS expression is evaluated as signed -1.