fix for error message in recursive mode for an empty folder

-r on empty directory resulted in zstd waiting input from stdin. now zstd exits without error and prints a warning message explaining why no processing happened (no files or directories to process).

programs/zstdcli.c

@@ -834,6 +834,7 @@ int main(int argCount, const char* argv[])
     size_t streamSrcSize = 0;
     size_t targetCBlockSize = 0;
     size_t srcSizeHint = 0;
+    size_t nbInputFileNames = 0;
     int dictCLevel = g_defaultDictCLevel;
     unsigned dictSelect = g_defaultSelectivityLevel;
 #ifndef ZSTD_NODICT
@@ -1256,6 +1257,8 @@ int main(int argCount, const char* argv[])
         }
     }
 
+    nbInputFileNames = filenames->tableSize; /* saving number of input files */
+
     if (recursive) {  /* at this stage, filenameTable is a list of paths, which can contain both files and directories */
         UTIL_expandFNT(&filenames, followLinks);
     }
@@ -1358,7 +1361,17 @@ int main(int argCount, const char* argv[])
 #endif
 
     /* No input filename ==> use stdin and stdout */
-    if (filenames->tableSize == 0) UTIL_refFilename(filenames, stdinmark);
+    if (filenames->tableSize == 0) {
+      /* It is possible that the input
+       was a number of empty directories. In this case
+       stdin and stdout should not be used */
+       if (nbInputFileNames > 0 ){
+        DISPLAYLEVEL(2, "please provide correct input file(s) or non-empty directories -- ignored \n");
+        CLEAN_RETURN(2);
+       }
+       UTIL_refFilename(filenames, stdinmark);
+    }
+
     if (!strcmp(filenames->fileNames[0], stdinmark) && !outFileName)
         outFileName = stdoutmark;  /* when input is stdin, default output is stdout */