Fix printf-family functions on long double inf (#1273)

Cosmopolitan's printf-family functions currently very poorly handle
being passed a long double infinity.

For instance, a program such as:

```cpp
#include <stdio.h>

int main()
{
    printf("%f\n", 1.0 / 0.0);
    printf("%Lf\n", 1.0L / 0.0L);
    printf("%e\n", 1.0 / 0.0);
    printf("%Le\n", 1.0L / 0.0L);
    printf("%g\n", 1.0 / 0.0);
    printf("%Lg\n", 1.0L / 0.0L);
}
```

will currently output the following:

```
inf
0.000000[followed by 32763 more zeros]
inf
N.aN0000e-32769
inf
N.aNe-32769
```

when the correct expected output would be:

```
inf
inf
inf
inf
inf
inf
```

This patch fixes this, and adds tests for the behavior.

Author: GabrielRavier

libc/stdio/fmt.c

@@ -615,7 +615,7 @@ static void __fmt_ldfpbits(union U *u, struct FPBits *b) {
 #if LDBL_MANT_DIG == 113
       b->bits[3] |= 1 << (112 - 32 * 3);  // set lowest exponent bit
 #endif
-    } else if (b->bits[0] | b->bits[1] | b->bits[2] | b->bits[3]) {
+    } else if (isnan(u->ld)) {
       i = STRTOG_NaN;
     } else {
       i = STRTOG_Infinite;
@@ -1145,8 +1145,8 @@ int __fmt(void *fn, void *arg, const char *format, va_list va, int *wrote) {
           s = s0 =
               gdtoa(fpb.fpi, fpb.ex, fpb.bits, &fpb.kind, 3, prec, &decpt, &se);
         }
-        if (decpt == 9999) {
-        Format9999:
+        if (decpt == 9999 || decpt == -32768) {
+        FormatDecpt9999Or32768:
           if (s0)
             freedtoa(s0);
           bzero(special, sizeof(special));
@@ -1258,8 +1258,8 @@ int __fmt(void *fn, void *arg, const char *format, va_list va, int *wrote) {
           s = s0 = gdtoa(fpb.fpi, fpb.ex, fpb.bits, &fpb.kind, prec ? 2 : 0,
                          prec, &decpt, &se);
         }
-        if (decpt == 9999)
-          goto Format9999;
+        if (decpt == 9999 || decpt == -32768)
+          goto FormatDecpt9999Or32768;
         c = se - s;
         prec1 = prec;
         if (!prec) {
@@ -1304,8 +1304,8 @@ int __fmt(void *fn, void *arg, const char *format, va_list va, int *wrote) {
           s = s0 = gdtoa(fpb.fpi, fpb.ex, fpb.bits, &fpb.kind, prec ? 2 : 0,
                          prec, &decpt, &se);
         }
-        if (decpt == 9999)
-          goto Format9999;
+        if (decpt == 9999 || decpt == -32768)
+          goto FormatDecpt9999Or32768;
       FormatExpo:
         if (fpb.sign /* && (x || sign) */)
           sign = '-';
@@ -1386,7 +1386,7 @@ int __fmt(void *fn, void *arg, const char *format, va_list va, int *wrote) {
         }
         if (fpb.kind == STRTOG_Infinite || fpb.kind == STRTOG_NaN) {
           s0 = 0;
-          goto Format9999;
+          goto FormatDecpt9999Or32768;
         }
         prec1 = __fmt_fpiprec(&fpb);
         if ((flags & FLAGS_PRECISION) && prec < prec1) {

test/libc/stdio/snprintf_test.c

@@ -35,3 +35,39 @@ TEST(snprintf, testPlusFlagOnChar) {
   ASSERT_EQ(i, 1);
   ASSERT_STREQ(buf, "=");
 }
+
+TEST(snprintf, testInf) {
+  char buf[10] = {};
+  int i = snprintf(buf, sizeof(buf), "%f", 1.0 / 0.0);
+
+  ASSERT_EQ(i, 3);
+  ASSERT_STREQ(buf, "inf");
+
+  memset(buf, 0, 4);
+  i = snprintf(buf, sizeof(buf), "%Lf", 1.0L / 0.0L);
+  ASSERT_EQ(i, 3);
+  ASSERT_STREQ(buf, "inf");
+
+  memset(buf, 0, 4);
+  i = snprintf(buf, sizeof(buf), "%e", 1.0 / 0.0);
+  ASSERT_EQ(i, 3);
+  ASSERT_STREQ(buf, "inf");
+
+  memset(buf, 0, 4);
+  i = snprintf(buf, sizeof(buf), "%Le", 1.0L / 0.0L);
+  ASSERT_EQ(i, 3);
+  ASSERT_STREQ(buf, "inf");
+
+  memset(buf, 0, 4);
+  i = snprintf(buf, sizeof(buf), "%g", 1.0 / 0.0);
+  ASSERT_EQ(i, 3);
+  ASSERT_STREQ(buf, "inf");
+
+  memset(buf, 0, 4);
+  i = snprintf(buf, sizeof(buf), "%Lg", 1.0L / 0.0L);
+  ASSERT_EQ(i, 3);
+  ASSERT_STREQ(buf, "inf");
+
+  for (i = 4; i < 10; ++i)
+    ASSERT_EQ(buf[i], '\0');
+}