Fix panic in FormFile if file not found

[lukesolo]

func (c *context) FormFile(name string) (*multipart.FileHeader, error) {
	f, fh, err := c.request.FormFile(name)
	defer f.Close()
	return fh, err
}

f.Close() panics with nil pointer exception if error wasn't checked.

[codecov]

Codecov Report

Merging #1515 into master will decrease coverage by 0.07%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #1515      +/-   ##
==========================================
- Coverage   84.91%   84.83%   -0.08%     
==========================================
  Files          28       28              
  Lines        2161     2163       +2     
==========================================
  Hits         1835     1835              
- Misses        212      213       +1     
- Partials      114      115       +1

Impacted Files	Coverage Δ
context.go	89.72% <33.33%> (-0.72%)	⬇️

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