Make detection of symbolic links more consistent

While fixing the detection of symbolic links on OpenBSD I noticed
inconsistent behaviour:

$ echo hello > hello
$ ln -s hello world
$ zstd hello world
Warning : world is a symbolic link, ignoring
hello                :316.67%   (     6 =>     19 bytes, hello.zst
$ ls *.zst
hello.zst
$ zstd world
world                :316.67%   (     6 =>     19 bytes, world.zst)
$ ls *.zst
hello.zst world.zst

programs/zstdcli.c

@@ -951,6 +951,8 @@ int main(int argCount, const char* argv[])
                 filenameTable[fileNamesNb++] = filenameTable[u];
             }
         }
+        if (fileNamesNb == 0 && filenameIdx > 0)
+            CLEAN_RETURN(1);
         filenameIdx = fileNamesNb;
     }
     if (recursive) {  /* at this stage, filenameTable is a list of paths, which can contain both files and directories */