Change sprintf to snprintf

[wangkuiyi]

On macOS, sprintf has been deprecated. Building dumper.c will error if -Werror,-Wdeprecated-declaration is set.

/Users/y/w/libyaml/src/dumper.c:254:5: error: 'sprintf' is deprecated: This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead. [-Werror,-Wdeprecated-declarations]
    sprintf((char *)anchor, ANCHOR_TEMPLATE, anchor_id);
    ^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.1.sdk/usr/include/stdio.h:188:1: note: 'sprintf' has been explicitly marked deprecated here
__deprecated_msg("This function is provided for compatibility reasons only.  Due to security concerns inherent in the design of sprintf(3), it is highly recommended that you use snprintf(3) instead.")
^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX13.1.sdk/usr/include/sys/cdefs.h:215:48: note: expanded from macro '__deprecated_msg'
        #define __deprecated_msg(_msg) __attribute__((__deprecated__(_msg)))

[wangkuiyi]

A complete fix should cover unit tests and examples. If such fixes are welcome, I can do it in a separate pull request.