app: in GimpBacktrace Windows backend, avoid bogus symbol addresses

In the GimpBacktrace Windows backend, avoid reporting meaningless
symbol addresses when failing to retrieve meaningful ones.
Unfortunately, it seems that we never get symbol addresses for
symbols that have debug information, which negatively affects the
log viewer's call graph.  We're going to have to work around this.

(cherry picked from commit 52772cf3ff)

app/core/gimpbacktrace-windows.c

@@ -668,7 +668,7 @@ gimp_backtrace_get_address_info (guintptr                  address,
       g_strlcpy (info->symbol_name, symbol_info->Name,
                  sizeof (info->symbol_name));
 
-      info->symbol_address = address - offset;
+      info->symbol_address = offset ? address - offset : 0;
 
       result = TRUE;
     }