girnode: Improve int types in GIIrNodeField

`GIIrNodeField` is built dynamically at runtime (rather than being
mmapped from disk), so its types can accurately reflect their runtime
semantics, rather than an on-disk format.

As part of this, switch from `atoi()` to `g_ascii_string_to_unsigned()`
for parsing the relevant fields from a GIR XML file. This means we now
get error handling for invalid integers.

This also includes some offset validity changes which were forgotten
from commit 515b3fc1dc.

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

Helps: #3155

girepository/girnode.c

@@ -1601,7 +1601,7 @@ gi_ir_node_build_typelib (GIIrNode         *node,
         blob->writable = field->writable;
         blob->reserved = 0;
         blob->bits = 0;
-        if (field->offset >= 0)
+        if (field->offset_state == GI_IR_OFFSETS_COMPUTED)
           blob->struct_offset = field->offset;
         else
           blob->struct_offset = 0xFFFF; /* mark as unknown */