config_format: cf_yaml: Align the behavior of dirname against POSIX

When nothing found of directory separators on the given argument of
dirname(), we need to align this behavior.
After aligning the behavior, we can proceed to load configurations from specified workdir
folders.

Signed-off-by: Hiroshi Hatake <hiroshi@chronosphere.io>

Author: cosmo0920

src/config_format/flb_cf_yaml.c

@@ -537,8 +537,10 @@ static char *dirname(char *path)
     ptr = strrchr(path, '\\');
 
     if (ptr == NULL) {
-        return path;
+        /* No directory component */
+        return ".";
     }
+
     *ptr++='\0';
     return path;
 }