checker: fix nested struct reference type field initialized check. (fix: #15741) (#15752)

vlib/x/json2/decoder.v

@@ -87,7 +87,7 @@ pub fn (err UnknownTokenError) msg() string {
 
 struct Parser {
 mut:
-	scanner      &Scanner
+	scanner      &Scanner = unsafe { nil }
 	p_tok        Token
 	tok          Token
 	n_tok        Token

vlib/x/ttf/render_bmp.v

@@ -18,8 +18,8 @@ import math
 
 pub struct BitMap {
 pub mut:
-	tf       &TTF_File
-	buf      &u8 = unsafe { 0 } // pointer to the memory buffer
+	tf       &TTF_File = unsafe { nil }
+	buf      &u8       = unsafe { nil } // pointer to the memory buffer
 	buf_size int // allocated buf size in bytes
 	width    int = 1 // width of the buffer
 	height   int = 1 // height of the buffer

vlib/x/ttf/render_sokol_cpu.v

@@ -19,7 +19,7 @@ import sokol.gfx
 
 pub struct TTF_render_Sokol {
 pub mut:
-	bmp &BitMap // Base bitmap render
+	bmp &BitMap = unsafe { nil } // Base bitmap render
 	// rendering fields
 	sg_img       gfx.Image // sokol image
 	scale_reduct f32 = 2.0 // scale of the cpu texture for filtering