console: initialize buf_size before calling open_memstream

POSIX open_memstream documentation [1] does not mention that it may use
the  value supplied in buf_size before the call.
newlib implementation of open_memstream does use it as a hint of the
buffer size [2]. To avoid using potential garbage in this variable,
newlib caps the size to 64kB (!).

If the allocation of this initial buffer fails, NULL file pointer is
returned. Previous code did not check returned file pointer and crashed
when it was used.

Initialize size to zero (in which case newlib allocates a 64 byte
buffer), and check the returned file pointer.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/functions/open_memstream.html
[2] https://github.com/espressif/newlib-esp32/blob/23c0f21/newlib/libc/stdio/open_memstream.c#L26-L29
    https://github.com/espressif/newlib-esp32/blob/23c0f21/newlib/libc/stdio/open_memstream.c#L324-L336

components/console/commands.c

@@ -105,11 +105,13 @@ esp_err_t esp_console_cmd_register(const esp_console_cmd_t *cmd)
         asprintf(&item->hint, " %s", cmd->hint);
     } else if (cmd->argtable) {
         /* Generate hint based on cmd->argtable */
-        char* buf;
-        size_t buf_size;
+        char* buf = NULL;
+        size_t buf_size = 0;
         FILE* f = open_memstream(&buf, &buf_size);
-        arg_print_syntax(f, cmd->argtable, NULL);
-        fclose(f);
+        if (f != NULL) {
+            arg_print_syntax(f, cmd->argtable, NULL);
+            fclose(f);
+        }
         item->hint = buf;
     }
     item->argtable = cmd->argtable;