Extend series before assign the last character

Or the last character will be missing, as tail is not set before
extending and Extend_Series only copy content up to "tail", and
terminate the new series, which clears the newly assigned value
(by "chr").

Fixes Atronix bug rebol#85

src/core/l-scan.c

@@ -457,10 +457,10 @@
 
 		src++;
 
-		*UNI_SKIP(buf, buf->tail) = chr;
-
 		if (SERIES_LEN(buf) >= SERIES_REST(buf)) Extend_Series(buf, 1);
 
+		*UNI_SKIP(buf, buf->tail) = chr;
+
 		buf->tail ++;
     }