binarytree._populate: suppress PORTAGE_SSH_OPTS KeyError (bug 572494)

Use the "get" method to avoid triggering a KeyError message.

Fixes: 39d81c514c33 ("[...]config.__getitem__(): Partially drop backward compatibility for nonexistent keys.")
X-Gentoo-Bug: 572494
X-Gentoo-Bug-url: https://bugs.gentoo.org/show_bug.cgi?id=572494
(cherry picked from commit 90e142b8fccba0507687b747478c5e060effcb00)

BUG=chromium:462099
TEST=updating sdk binpkgs doesn't throw errors

Change-Id: Ie5c302604606093572a8df6536bc7b7cee3fb659
Reviewed-on: https://chromium-review.googlesource.com/830293
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Chirantan Ekbote <chirantan@chromium.org>
diff --git a/pym/portage/dbapi/bintree.py b/pym/portage/dbapi/bintree.py
index 844d015..d391f38 100644
--- a/pym/portage/dbapi/bintree.py
+++ b/pym/portage/dbapi/bintree.py
@@ -960,10 +960,9 @@
 						}
 
 						for k in ("PORTAGE_SSH_OPTS",):
-							try:
-								fcmd_vars[k] = self.settings[k]
-							except KeyError:
-								pass
+							v = self.settings.get(k)
+							if v is not None:
+								fcmd_vars[k] = v
 
 						success = portage.getbinpkg.file_get(
 							fcmd=fcmd, fcmd_vars=fcmd_vars)