libsysprof-analyze: handle NULL process info

This can happen for "process 0" for example, or anything that was
recorded for a pid which did not get a SysprofCaptureProcess frame.

src/libsysprof-analyze/sysprof-bundled-symbolizer.c

@@ -181,7 +181,7 @@ sysprof_bundled_symbolizer_symbolize (SysprofSymbolizer        *symbolizer,
   const Decoded key = {
     .addr_begin = address,
     .addr_end = address,
-    .pid = process_info->pid,
+    .pid = process_info ? process_info->pid : 0,
     .offset = 0,
     .tag_offset = 0,
   };