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

src/unwind.c

@@ -28,7 +28,7 @@
 
 #include "unwinddefs.h"
 
-UNSIGNED N_OP_unwind(register LispPTR *cstkptr, register LispPTR tos, int n, int keep) {
+LispPTR *N_OP_unwind(register LispPTR *cstkptr, register LispPTR tos, int n, int keep) {
   register int num;           /* number of UNBOUND slot */
   register LispPTR *endptr;   /* unwind limit */
   register LispPTR *lastpvar; /* points PVar slot that is unbounded. */
@@ -64,7 +64,10 @@ UNSIGNED N_OP_unwind(register LispPTR *cstkptr, register LispPTR tos, int n, int
 
   if (endptr > cstkptr) {
     CurrentStackPTR = (DLword *)cstkptr;
-    ERROR_EXIT(tos);
+    /* this would be ERROR_EXIT(tos); but for having to return a pointer */
+    TopOfStack = tos;
+    Error_Exit = 1;
+    return (LispPTR *)(-1);
   }
   *cstkptr++ = tos;
 
@@ -85,7 +88,7 @@ UNSIGNED N_OP_unwind(register LispPTR *cstkptr, register LispPTR tos, int n, int
   /* endptr = cstkptr */
 
   if (keep) { *(cstkptr++) = tos; }
-  return ((UNSIGNED)cstkptr);
+  return (cstkptr);
 
 } /* N_OP_unwind */