FIX: better handling missing padding of the DEBASE (64) input

Now it is handled only when the input is as URL safe variant.

Relates to: metaeducation/rebol-issues#2318

Possible test:

```
binary? try [debase {
	4BzEEOtIpmVdVEZNCqS7baC4crd0pqnRH_5IB3jw3bcxGn6QLvnEtfdUdi
	YrqBdss1l58BQ3KhooKeQTa9AB0Hw_Py5PJdTJNPY8cQn7ouZ2KKDcmnPG
	BY5t7yLc1QlQ5xHdwW1VhvKn-nXqhJTBgIPgtldC-KDV5z-y2XDwGUc
}] ;== should be TRUE

```

Author: Oldes

src/core/f-enbase.c

@@ -486,10 +486,19 @@
 		}
 	}
 
-	if(flip==2) {
-		// input may not be padded... see https://github.com/rebol/rebol-issues/issues/2318 
-		*bp++ = (REBYTE)(accum >> 4);
-	} else if (flip) goto err;
+	if (flip) {
+		if(urlSafe) {
+			// in url safe mode may be ommited the final inut padding
+			if (flip == 3) {
+				*bp++ = (REBYTE)(accum >> 10);
+				*bp++ = (REBYTE)(accum >> 2);
+			}
+			else if (flip == 2) {
+				*bp++ = (REBYTE)(accum >> 4);
+			}
+			else goto err;
+		} else goto err;
+	}
 
 	*bp = 0;
 	ser->tail = bp - STR_HEAD(ser);