Fix panics in GetConditionalFormats on malformed conditional formatting rules

[arpitjain099]

Fix for GHSA-rxcj-4pj5-74gr. Thanks for asking for the PR.

What panics

GetConditionalFormats reaches three extractors that index cfRule sub-elements straight out of the worksheet XML, without checking length and in one case without checking for nil. A workbook whose rule is missing a child that Excel would always write panics the call, so a service that opens an untrusted spreadsheet and reads its conditional formats can be crashed by a small file.

Four sinks, all in styles.go:

where	guard before	what reaches it
extractCondFmtCellIs	none	cellIs rule with no <formula>, indexing Formula[0] on an empty slice
extractCondFmtColorScale	none	colorScale rule with no <colorScale> child, so c.ColorScale is nil
same, three-colour branch	colors == 3	three <color> elements and one <cfvo>, indexing Cfvo[1] and Cfvo[2]
extractCondFmtDataBar	c.DataBar != nil	empty <dataBar></dataBar>, indexing Cfvo[0], Cfvo[1] and Color[0]

The third one is not in the advisory. It turned up while writing the tests: that branch is gated on the colour count and then indexes the cfvo slice, so the two counts can disagree. Worth fixing in the same pass.

The change

Four guards, eleven lines in styles.go. Each one leaves the surrounding behaviour alone: a rule missing its children now yields the zero value for those fields rather than stopping the caller. No signature changes and no new error returns, so this is not a breaking change for anyone whose files are well formed.

Tests

Four cases appended to styles_test.go. Each builds a minimal workbook in memory with archive/zip, opens it with the public OpenReader and calls GetConditionalFormats, so they drive the same path a caller does rather than reaching into internals.

Verified individually, since a panic takes the test binary down and would otherwise mask the later cases:

cellIs_without_formula         panic: index out of range [0] with length 0
colorScale_element_absent      panic: invalid memory address or nil pointer dereference
dataBar_element_empty          panic: index out of range [0] with length 0
three_colors_but_one_cfvo      panic: index out of range [1] with length 1

With the change, all four pass and the full suite is green:

ok  github.com/xuri/excelize/v2  72.413s

gofmt and go vet are clean.

Happy to split the third fix into its own commit or PR if you would rather keep the advisory scope exact.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.62%. Comparing base (ab49f6f) to head (2239a97).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2375   +/-   ##
=======================================
  Coverage   99.62%   99.62%           
=======================================
  Files          32       32           
  Lines       27001    27004    +3     
=======================================
+ Hits        26899    26902    +3     
  Misses         53       53           
  Partials       49       49           

Flag	Coverage Δ
unittests	99.62% <100.00%> (+<0.01%)	⬆️

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[xuri]

Thanks for your contribution. I've made some changes based on your branch for simplify unit tests code.