Verify game state and configuration reference sanity

Table elements reference other table elements.  Bad things happen when
references dangle.  ef_verify() already checks whether the referenced
table elements exist.  This commit makes it check whether the elements
are "in use".  This catches stuff like living planes on dead carriers.

src/lib/common/ef_verify.c

@@ -122,6 +122,13 @@ verify_tabref(int type, int row, struct castr *ca, int idx, long val)
 			val, ef_nameof(tabno), ef_nelem(tabno));
 	    return -1;
 	}
+	/* laziness: assumes TABNO is EFF_MEM */
+	if (val >= 0 && !empobj_in_use(tabno, ef_ptr(tabno, val))) {
+	    verify_fail(type, row, ca, idx,
+			"value %ld refers to missing element of table %s",
+			val, ef_nameof(tabno));
+	    return -1;
+	}
     }
     return 0;
 }