cgen: fix struct field init within fn without result type (fix #24970) (#24973)

vlib/v/gen/c/cgen.v

@@ -7239,10 +7239,10 @@ fn (mut g Gen) or_block(var_name string, or_block ast.OrExpr, return_type ast.Ty
 				g.writeln('\t${styp} ${err_obj} = {0};')
 				if g.fn_decl.return_type.has_flag(.result) {
 					g.writeln('\t${err_obj}.is_error = true;')
+					g.writeln('\t${err_obj}.err = ${cvar_name}${tmp_op}err;')
 				} else if g.fn_decl.return_type.has_flag(.option) {
 					g.writeln('\t${err_obj}.state = 2;')
 				}
-				g.writeln('\t${err_obj}.err = ${cvar_name}${tmp_op}err;')
 				g.writeln('\treturn ${err_obj};')
 			}
 		}

vlib/v/gen/c/testdata/struct_field_result_init.c.must_have

@@ -0,0 +1,5 @@
+_result_math__big__Integer _t1 = math__big__integer_from_string(_S("115792089237316195423570985008687907853269984665640564039457584007913129639747"));
+if (_t1.is_error) {
+math__big__Integer _t2 = {0};
+return _t2;
+}

vlib/v/gen/c/testdata/struct_field_result_init.vv

@@ -0,0 +1,32 @@
+import math
+import math.big { Integer }
+import crypto.rand
+
+struct SSS {
+mut:
+	prime Integer = big.integer_from_string('115792089237316195423570985008687907853269984665640564039457584007913129639747')!
+}
+
+// random returns a random number from the range (0, prime-1) inclusive
+fn (s SSS) random() !Integer {
+	mut result := big.zero_int + s.prime
+	result = result - big.one_int
+	return rand.int_big(result)
+}
+
+// mod_inverse computes the multiplicative inverse of the number on the field
+// prime; more specifically, number * inverse == 1; Note: number should never be
+// zero
+fn mod_inverse(number Integer) Integer {
+	s := SSS{}
+	copy := number % s.prime
+	pcopy := s.prime
+
+	_, _, y := math.egcd(pcopy.int(), copy.int())
+
+	return (s.prime + big.integer_from_i64(y)) % s.prime
+}
+
+fn main() {
+	mod_inverse(big.integer_from_string('1')!)
+}