Fix psql crash while parsing SQL file whose encoding is different from

client encoding and the client encoding is not *safe* one. Such an
example is, file encoding is UTF-8 and client encoding SJIS. Patch
contributed by Jiang Guiqing.

src/bin/psql/psqlscan.l

@@ -1806,7 +1806,7 @@ prepare_buffer(const char *txt, int len, char **txtcopy)
 			/* first byte should always be okay... */
 			newtxt[i] = txt[i];
 			i++;
-			while (--thislen > 0)
+			while (--thislen > 0 && i < len)
 				newtxt[i++] = (char) 0xFF;
 		}
 	}