Fix crash in test-engine on Arch

Thanks, Valgrind:

==515314== Invalid read of size 8
==515314==    at 0x4ED46F3: gncInvoiceRemoveEntries (gncInvoice.c:767)
==515314==    by 0x142B35: teardown_with_invoice (utest-Invoice.c:274)
...
==515314==  Address 0x8557b98 is 8 bytes inside a block of size 24 free'd
==515314==    at 0x484620F: free (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==515314==    by 0x51B565D: g_list_remove (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7600.1)
==515314==    by 0x4ED42EF: gncInvoiceRemoveEntry (gncInvoice.c:688)
==515314==    by 0x4ED46A2: gncInvoiceRemoveEntries (gncInvoice.c:781)
==515314==    by 0x142B35: teardown_with_invoice (utest-Invoice.c:274)
...
==515314==  Block was alloc'd at
==515314==    at 0x4843828: malloc (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==515314==    by 0x51BD948: g_malloc (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7600.1)
==515314==    by 0x51B1CB9: ??? (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7600.1)
==515314==    by 0x4ED4271: gncInvoiceAddEntry (gncInvoice.c:676)
==515314==    by 0x142401: setup_with_invoice (utest-Invoice.c:142)
...

ok 57 /engine/gncInvoice/post trans - vendor bill

libgnucash/engine/gncInvoice.c

@@ -760,12 +760,13 @@ void gncInvoiceSortEntries (GncInvoice *invoice)
 
 void gncInvoiceRemoveEntries (GncInvoice *invoice)
 {
-    GList *node;
-
     if (!invoice) return;
 
-    for (node = invoice->entries; node; node = node->next)
+    // gnc{Bill,Invoice}RemoveEntry free the "entry" node.
+    // Make sure to save "next" first.
+    for (GList *next, *node = invoice->entries; node; node = next)
     {
+        next = node->next;
         GncEntry *entry = node->data;
 
         switch (gncInvoiceGetOwnerType (invoice))