svg_loader: If there is already set color url, it will be deleted.

svg_loader: If there is already set color url, it will be deleted.

When setting the url for color, if there is an already set url,
it will be overwritten after deletion. This prevents memory leaks.

asan result)

Direct leak of 2 byte(s) in 1 object(s) allocated from:
    #0 0x7ff1d547bc68 in __interceptor_malloc (/usr/lib/x86_64-linux-gnu/libasan.so.5+0x10bc68)
    #1 0x7ff1d4e435b4 in _idFromUrl ../src/loaders/svg/tvgSvgLoader.cpp:327
    #2 0x7ff1d4e44a8e in _toColor ../src/loaders/svg/tvgSvgLoader.cpp:558
    #3 0x7ff1d4e42ee9 in _parseStyleAttr ../src/loaders/svg/tvgSvgLoader.cpp:1033
    #4 0x7ff1d4e7f092 in simpleXmlParseAttributes(char const*, unsigned int, bool (*)(void*, char const*, char const*), void const*) ../src/loaders/svg/tvgXmlParser.cpp:361
    #5 0x7ff1d4e597d3 in _createPathNode ../src/loaders/svg/tvgSvgLoader.cpp:1363
    #6 0x7ff1d4e61359 in _svgLoaderParserXmlOpen ../src/loaders/svg/tvgSvgLoader.cpp:2723
    #7 0x7ff1d4e61c49 in _svgLoaderParser ../src/loaders/svg/tvgSvgLoader.cpp:2801
    #8 0x7ff1d4e7f3f6 in simpleXmlParse(char const*, unsigned int, bool, bool (*)(void*, SimpleXMLType, char const*, unsigned int), void const*) ../src/loaders/svg/tvgXmlParser.cpp:429
    #9 0x7ff1d4e639a1 in SvgLoader::run(unsigned int) ../src/loaders/svg/tvgSvgLoader.cpp:3121
    #10 0x7ff1d4dc8b75 in tvg::Task::operator()(unsigned int) ../src/lib/tvgTaskScheduler.h:68
    #11 0x7ff1d4dc8b75 in tvg::TaskSchedulerImpl::run(unsigned int) ../src/lib/tvgTaskScheduler.cpp:138
    #12 0x7ff1d4dc98f7 in tvg::TaskSchedulerImpl::TaskSchedulerImpl(unsigned int)::{lambda()#1}::operator()() const ../src/lib/tvgTaskScheduler.cpp:113
    #13 0x7ff1d4dc98f7 in void std::__invoke_impl<void, tvg::TaskSchedulerImpl::TaskSchedulerImpl(unsigned int)::{lambda()#1}>(std::__invoke_other, tvg::TaskSchedulerImpl::TaskSchedulerImpl(unsigned int)::{lambda()#1}&&) /usr/include/c++/9/bits/invoke.h:60
    #14 0x7ff1d4dc98f7 in std::__invoke_result<tvg::TaskSchedulerImpl::TaskSchedulerImpl(unsigned int)::{lambda()#1}>::type std::__invoke<tvg::TaskSchedulerImpl::TaskSchedulerImpl(unsigned int)::{lambda()#1}>(std::__invoke_result&&, (tvg::TaskSchedulerImpl::TaskSchedulerImpl(unsigned int)::{lambda()#1}&&)...) /usr/include/c++/9/bits/invoke.h:95
    #15 0x7ff1d4dc98f7 in void std::thread::_Invoker<std::tuple<tvg::TaskSchedulerImpl::TaskSchedulerImpl(unsigned int)::{lambda()#1}> >::_M_invoke<0ul>(std::_Index_tuple<0ul>) /usr/include/c++/9/thread:244
    #16 0x7ff1d4dc98f7 in std::thread::_Invoker<std::tuple<tvg::TaskSchedulerImpl::TaskSchedulerImpl(unsigned int)::{lambda()#1}> >::operator()() /usr/include/c++/9/thread:251
    #17 0x7ff1d4dc98f7 in std::thread::_State_impl<std::thread::_Invoker<std::tuple<tvg::TaskSchedulerImpl::TaskSchedulerImpl(unsigned int)::{lambda()#1}> > >::_M_run() /usr/include/c++/9/thread:195
    #18 0x7ff1d3a344bf  (/usr/lib/x86_64-linux-gnu/libstdc++.so.6+0xd44bf)

example)
<svg height="400" width="600" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg">
   <defs>
      <radialGradient r="12" fy="10" fx="10" cy="10" cx="10" id="i" xlink:href="#d"/>
      <clipPath id="g" clipPathUnits="userSpaceOnUse">
         <path d="M0 10 10z" style="marker:none" fill="#fff" stroke-width="2"/>
      </clipPath>
   </defs>
         <path d="M0 10 Z" fill="url(#g)" style="fill:url(#i);fill-opacity:1"/>
</svg>

Change-Id: I14c605e7555dbda19113b9f9027954654e423f4b