gdesktopappinfo: Fix a maybe-uninitialized warning

scan-build thinks that `term_arg` could be used uninitialised. I think
there isn’t a bug here because that use is protected by the
`found_terminal == NULL` check and early return. But perhaps that logic
is a bit too complex for static analysis, so add a default value for the
variable.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #1767

gio/gdesktopappinfo.c

@@ -2687,7 +2687,7 @@ prepend_terminal_to_vector (int          *argc,
   size_t term_argc;
   char *found_terminal;
   char **the_argv;
-  const char *term_arg;
+  const char *term_arg = NULL;
   static const struct {
     const char *exec;
     const char *exec_arg;