lua: fix buffer underrun

A line containing just "\n" will cause a buffer underrun.

Test:
tshark -r test/captures/sipmsg.log -X lua_script:test/lua/acme_file.lua -w /dev/null

Caught by ASAN.

Change-Id: Idf38f1af2b211101b3929ee3fbd83c54c99c1e21
Reviewed-on: https://code.wireshark.org/review/1673
Reviewed-by: Evan Huus <eapache@gmail.com>

epan/wslua/wslua_file.c

@@ -192,7 +192,7 @@ static int File_read_line(lua_State *L, FILE_T ft) {
     if (linebuff[length-1] == '\n') {
         length--;
         /* Nor do we want '\r' (as will be written when log is created on windows) */
-        if (linebuff[length-1] == '\r') {
+        if (length > 0 && linebuff[length - 1] == '\r') {
             length--;
         }
         linebuff[length] = '\0';