std.fmt: Fix incorrect behavior with large floating point integers.

I consider this an interim workaround/hack until ziglang#1299 is finished.

There is a bug in the original C implementation of the errol3 (and errol4)
algorithm that can result in undefined behavior or an obviously incorrect
result (leading ':' in the output)

This change checks for those two problems and uses a slower fallback
path if they occur. I can't guarantee that this will always produce
the correct result, but since the workaround is only used if the original
algorithm is guaranteed to fail, it should never turn a previously-correct
result into an incorrect one.

Fixes ziglang#11283

lib/std/fmt.zig

@@ -2299,6 +2299,8 @@ test "float.decimal" {
     try expectFmt("f64: 0.00000", "f64: {d:.5}", .{@as(f64, 1.40130e-45)});
     try expectFmt("f64: 0.00000", "f64: {d:.5}", .{@as(f64, 9.999960e-40)});
     try expectFmt("f64: 10000000000000.00", "f64: {d:.2}", .{@as(f64, 9999999999999.999)});
+    try expectFmt("f64: 10000000000000000000000000000000000000", "f64: {d}", .{@as(f64, 1e37)});
+    try expectFmt("f64: 100000000000000000000000000000000000000", "f64: {d}", .{@as(f64, 1e38)});
 }
 
 test "float.libc.sanity" {

lib/std/fmt/errol.zig

@@ -106,7 +106,10 @@ fn errol3u(val: f64, buffer: []u8) FloatDecimal {
     } else if (val >= 16.0 and val < 9.007199254740992e15) {
         return errolFixed(val, buffer);
     }
+    return errolSlow(val, buffer);
+}
 
+fn errolSlow(val: f64, buffer: []u8) FloatDecimal {
     // normalize the midpoint
 
     const e = math.frexp(val).exponent;
@@ -336,7 +339,9 @@ fn errolInt(val: f64, buffer: []u8) FloatDecimal {
     var buf_index = u64toa(m64, buffer) - 1;
 
     if (mi != 0) {
-        buffer[buf_index - 1] += @boolToInt(buffer[buf_index] >= '5');
+        const round_up = buffer[buf_index] >= '5';
+        if (buf_index == 0 or (round_up and buffer[buf_index - 1] == '9')) return errolSlow(val, buffer);
+        buffer[buf_index - 1] += @boolToInt(round_up);
     } else {
         buf_index += 1;
     }