Fix pdfium-i#236: memory leaks from RenderPdf

- add FPDFAvail_Destroy(pdf_avail) on the early return path in RenderPdf

TBR=thestig@chromium.org
BUG=pdfium:236

Review URL: https://codereview.chromium.org/1410333007 .

(cherry picked from commit 0223be82e0afcfee7c501ad2754230353bff79ff)

Review URL: https://codereview.chromium.org/1414913003 .
1 file changed