Correct handling of return value of N_OP_unwind()

The return value of N_OP_unwind is really of type LispPTR *, however it was
declared as UNSIGNED (effectively uintptr_t) so that the ERROR_EXIT macro
could be used to return an error indication (-1, =UINT_MAX). The call site
checked for the error condition with (int)result < 0, not accounting for the
case where a native pointer may have the high-order bit set.

We correct this problem by declaring the return type as LispPTR *,
and expand the ERROR_EXIT macro in place substituting (LispPTR *)(-1)
as the error return value, and at the single call site check for equality
with that same value.

The test case was executing the opcode tester on a Raspberry Pi or
a BeagleBone Black/Debian 10.3 where the non-error result was > 0xB0000000

  	modified:   inc/inlineC.h
        modified:   inc/unwinddefs.h
        modified:   src/unwind.c

inc/inlineC.h

@@ -852,8 +852,7 @@ nextop2;								\
 
 #define UNWIND(n, m)							\
 		{	 						\
-		 if ((INT)(CSTKPTRL = (LispPTR *)			\
-		      N_OP_unwind(CSTKPTR, TOPOFSTACK, n, m)) < 0) 	\
+                    if ((CSTKPTRL = N_OP_unwind(CSTKPTR, TOPOFSTACK, n, m)) == (LispPTR *)-1) \
 			goto unwind_err;				\
 		 POP;							\
 		 nextop3;						\