Fix test mode and write to null

Fix test mode to not always return success.
Don't ask for permission to overwrite `nulmark`.

Before:

    > echo "hello world" > file
    > lz4 -t file
    successfully decoded 12 bytes
    > lz4 -tf file
    successfully decoded 12 bytes
    > lz4 file null
    Warning : /dev/null already exists
    Overwrite ? (Y/n) : n
    Error 12 : No. Operation aborted : /dev/null already exists
    > lz4 file /dev/null
    Warning : /dev/null already exists
    Overwrite ? (Y/n) : n
    Error 12 : No. Operation aborted : /dev/null already exists

After:

    > lz4 -t file
    Error 44 : Unrecognized header : file cannot be decoded
    > lz4 -tf file
    Error 44 : Unrecognized header : file cannot be decoded
    > lz4 file null
    Compressed 12 bytes into 31 bytes ==> 258.33%
    > lz4 file /dev/null
    Compressed 12 bytes into 31 bytes ==> 258.33%

1 files changed, 10 insertions, 2 deletions

diff --git a/programs/lz4io.c b/programs/lz4io.c
index f2d7b51..3a85830 100644
--- a/programs/lz4io.c
+++ b/programs/lz4io.c
@@ -136,6 +136,7 @@ static clock_t g_time = 0;
 *  Local Parameters
 **************************************/
 static int g_overwrite = 1;
+static int g_passThrough = 1;
 static int g_blockSizeId = LZ4IO_BLOCKSIZEID_DEFAULT;
 static int g_blockChecksum = 0;
 static int g_streamChecksum = 1;
@@ -184,6 +185,13 @@ int LZ4IO_setOverwrite(int yes)
    return g_overwrite;
 }
 
+/* Default setting : passThrough = 1; return : passThrough mode (0/1) */
+int LZ4IO_setPassThrough(int yes)
+{
+   g_passThrough = (yes!=0);
+   return g_passThrough;
+}
+
 /* blockSizeID : valid values : 4-5-6-7 */
 int LZ4IO_setBlockSizeID(int bsid)
 {
@@ -283,7 +291,7 @@ static int LZ4IO_getFiles(const char* input_filename, const char* output_filenam
     } else {
         /* Check if destination file already exists */
         *pfoutput=0;
-        if (output_filename != nulmark) *pfoutput = fopen( output_filename, "rb" );
+        if (strcmp(output_filename, nulmark)) *pfoutput = fopen( output_filename, "rb" );
         if (*pfoutput!=0) {
             fclose(*pfoutput);
             if (!g_overwrite) {
@@ -908,7 +916,7 @@ static unsigned long long selectDecoder(dRess_t ress, FILE* finput, FILE* foutpu
     EXTENDED_FORMAT;  /* macro extension for custom formats */
     default:
         if (nbCalls == 1) {  /* just started */
-            if (g_overwrite)
+            if (g_passThrough && g_overwrite)
                 return LZ4IO_passThrough(finput, foutput, MNstore);
             EXM_THROW(44,"Unrecognized header : file cannot be decoded");   /* Wrong magic number at the beginning of 1st stream */
         }