cext-index: propagate inline_scan error in `index_deref`

Before this change, revlog index corruption could be silently ignored in some
situation.

Differential Revision: https://phab.mercurial-scm.org/D8276

mercurial/cext/revlog.c

@@ -157,7 +157,10 b' static const char *index_deref(indexObje'
 			                             sizeof(*self->offsets));
 			if (self->offsets == NULL)
 				return (const char *)PyErr_NoMemory();
-			inline_scan(self, self->offsets);
+			Py_ssize_t ret = inline_scan(self, self->offsets);
+			if (ret == -1) {
+				return NULL;
+			};
 		}
 		return self->offsets[pos];
 	}