Make sure that the base64 decoder returns a terminated string.

(closes issue #10979)
Reported by: ys
Patches: 
      util.c.diff uploaded by ys (license 281)
	    - small mods by me


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@85543 65c4cc65-6c06-0410-ace0-fbb531ad65f3

main/utils.c

@@ -347,6 +347,9 @@ int ast_base64decode(unsigned char *dst, const char *src, int max)
 			cnt++;
 		}
 	}
+	if (cnt == max)
+		dst--;
+	*dst = '\0';
 	/* Dont worry about left over bits, they're extra anyway */
 	return cnt;
 }