Fix index overflow on data values array

[guillaume-ro-fr]

This is:

- [x] a bugfix

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

isMultiLevelSeries function throw an error on discontinuous indices : Notice: Undefined offset: 0.

[guillaume-ro-fr]

Example of XML :

...
<c:val>
	<c:numRef>
		<c:f>'Worksheet'!$F$32:$F$39</c:f>
		<c:numCache>
			<c:formatCode>0.0</c:formatCode>
			<c:ptCount val="8"/>
			<c:pt idx="4">
				<c:v>3.6</c:v>
			</c:pt>
			<c:pt idx="5">
				<c:v>6.5</c:v>
			</c:pt>
		</c:numCache>
	</c:numRef>
</c:val>
...

Note the index starting at 4, not 0.