pungi: Fix --nosource option

When running with this option, if a debuginfo package is included in the
final package set, there would be a crash due to missing SRPM. To
replicate this issue, just use the argument with input kickstart listing
a single package which has debuginfo.

This is not an issue for Pungi 4, as it never uses the option in the
first place.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

pungi/gather.py

@@ -1158,7 +1158,11 @@ class Pungi(PungiBase):
             self.add_debuginfo(po, msg)
 
             # flags
+            try:
                 srpm_po = self.sourcerpm_srpmpo_map[po.sourcerpm]
+            except:
+                self.logger.warning('Failed to find source for %s', po.sourcerpm)
+                srpm_po = None
             if srpm_po in self.input_packages:
                 self.input_packages.add(po)
             if srpm_po in self.fulltree_packages: