potential read access violation with an addon path setting

[anxdpanic]

Description

This fixes a potential read access violation when an add-on path setting source is empty.

Motivation and Context

Converted settings to new format and ran into this reproducible crash.
Settings similar to below would cause Kodi to crash on access, due to the empty <source/>

<?xml version="1.0" ?>
<settings version="1">
    <category help="" id="export" label="30001">
        <group id="1">
            <setting help="" id="export_path" label="30001" type="path">
                <level>0</level>
                <default/>
                <constraints>
                    <sources>
                        <source/>
                    </sources>
                    <allowempty>true</allowempty>
                </constraints>
                <control format="path" type="button">
                    <heading>30001</heading>
                </control>
            </setting>
        </group>
    </category>
</settings>

How Has This Been Tested?

Runtime, no longer crashes

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• My code follows the Code Guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the Contributing document
• I have added tests to cover my change
• All new and existing tests passed

[AlwinEsch]

Good fix!

[phunkyfish]

Your button @anxdpanic!