libs/libc/obstack: correctly append null byte at the end of string

obstack_printf and obstack_vprintf should terminate the C string with
null byte but lib_vsprintf doesn't do it. It must be done on top of
that unless we get unterminated string.

This is fix to be consistent with GlibC behavior.

This also includes minor tweak to use obstack_1grow directly instead of
calling obstack_puts.

libs/libc/obstack/lib_obstack_vprintf.c

@@ -54,8 +54,11 @@ static int obstack_puts(FAR struct lib_outstream_s *self,
 
 static void obstack_putc(FAR struct lib_outstream_s *self, int ch)
 {
-  char tmp = ch;
-  obstack_puts(self, &tmp, 1);
+  FAR struct obstack_stream *stream = (FAR struct obstack_stream *)self;
+
+  DEBUGASSERT(self);
+
+  obstack_1grow(stream->h, ch);
 }
 
 /****************************************************************************
@@ -93,5 +96,14 @@ int obstack_vprintf(FAR struct obstack *h, FAR const char *fmt, va_list ap)
   outstream.common.nput = 0;
   outstream.h = h;
 
-  return lib_vsprintf(&outstream.common, fmt, ap);
+  int nbytes = lib_vsprintf(&outstream.common, fmt, ap);
+
+  if (nbytes < 0)
+    {
+      obstack_free(h, obstack_finish(h));
+      return ERROR;
+    }
+
+  obstack_1grow(h, '\0');
+  return nbytes;
 }