strconv: fix error for string interpolation of float format (#13800)

vlib/strconv/format_mem.c.v

@@ -274,10 +274,13 @@ pub fn f64_to_str_lnd1(f f64, dec_digit int) string {
 
 		// println("r_i-d_pos: ${r_i - d_pos}")
 		if dot_res_sp >= 0 {
-			if (r_i - dot_res_sp) > dec_digit {
 			r_i = dot_res_sp + dec_digit + 1
-			}
 			res[r_i] = 0
+			for c1 in 1 .. dec_digit + 1 {
+				if res[r_i - c1] == 0 {
+					res[r_i - c1] = `0`
+				}
+			}
 			// println("result: [${tos(&res[0],r_i)}]")
 			tmp_res := tos(res.data, r_i).clone()
 			res.free()

vlib/v/tests/string_interpolation_float_fmt_test.v

@@ -0,0 +1,13 @@
+fn test_string_interpolation_float_fmt() {
+	mut a := 76.295
+	eprintln('${a:8.2}')
+	assert '${a:8.2}' == '   76.30'
+	eprintln('${a:8.2f}')
+	assert '${a:8.2f}' == '   76.30'
+
+	a = 76.296
+	eprintln('${a:8.2}')
+	assert '${a:8.2}' == '   76.30'
+	eprintln('${a:8.2f}')
+	assert '${a:8.2f}' == '   76.30'
+}