contrib/checkpatch: try to warn about uninitialized GError variables

When we have a GError* variable on the stack, we usually want to pass
it on to function that can fail. In that case, the variable MUST be
initialized to NULL. This is an easy mistake to make.

Note that this check still can have lots of false positives, for
example, if you have a struct with an GError field. In that case, you
would need to ensure that the entire struct is initialized. Ignore the
warning then.

Also, the check misses if you declare multiple variables on one line.
But that is already discouraged by our style.

contrib/scripts/checkpatch.pl

@@ -193,6 +193,7 @@ complain ("XXX marker are reserved for development while work-in-progress. Use T
 complain ("This gtk-doc annotation looks wrong") if $line =~ /\*.*\( *(transfer-(none|container|full)|allow none) *\) *(:|\()/;
 complain ("Prefer nm_assert() or g_return*() to g_assert*()") if $line =~ /g_assert/ and (not $filename =~ /\/tests\//) and (not $filename =~ /\/nm-test-/);
 complain ("Use gs_free_error with GError variables") if $line =~ /\bgs_free\b +GError *\*/;
+complain ("Initialize GError variables to NULL, if you pass them on") if $line =~ /\bGError +\*([a-z0-9_]+);/;
 complain ("Don't use strcmp/g_strcmp0 unless you need to sort. Consider nm_streq()/nm_streq0(),NM_IN_STRSET() for testing equality") if $line =~ /\b(strcmp|g_strcmp0)\b/;
 complain ("Don't use API that uses the numeric source id. Instead, use GSource and API like nm_g_idle_add(), nm_g_idle_add_source(), nm_clear_g_source_inst(), etc.") if $line =~ /\b(g_idle_add|g_idle_add_full|g_timeout_add|g_timeout_add_seconds|g_source_remove|nm_clear_g_source)\b/;
 complain ("Prefer g_snprintf() over snprintf() (for consistency)") if $line =~ /\b(snprintf)\b/;