lib/oe/package_manager: prevent testing an undefined variable

This prevents an error in do_populate_sdk when building
buildtools-tarball with ipk as the package manager:

Exception: UnboundLocalError: local variable 'pkg' referenced
before assignment

(From OE-Core rev: f971ae290cbbc51ec3669b57c55b2b7c0bd9bc87)

Signed-off-by: Joshua Lock <joshua.g.lock@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oe/package_manager.py

@@ -308,6 +308,7 @@ class PkgsList(object):
         output = dict()
         filename = ""
         dep = []
+        pkg = ""
         for line in cmd_output.splitlines():
             line = line.rstrip()
             if ':' in line: