extract: finish: fix bad assertion - ‘idx’ may be equal to ‘size’

‘idx’ is where _new_ data should be pushed into the buffer. Thus it is
perfectly valid for it to be equal to ‘size’ - it just means we need
to allocate more space before pushing data to it.

extract.c

@@ -72,11 +72,14 @@ extract_finish(struct extraction_context *ctx, char **text, size_t *len)
         ctx->buf[ctx->idx] = L'\0';
     } else {
         assert(ctx->idx > 0);
-        assert(ctx->idx < ctx->size);
+        assert(ctx->idx <= ctx->size);
         if (ctx->buf[ctx->idx - 1] == L'\n')
             ctx->buf[ctx->idx - 1] = L'\0';
-        else
+        else {
+            if (!ensure_size(ctx, 1))
+                goto out;
             ctx->buf[ctx->idx] = L'\0';
+        }
     }
 
     size_t _len = wcstombs(NULL, ctx->buf, 0);