Fix %c with nul character

Fixes #417

Author: jart

libc/fmt/fmt.c

@@ -335,12 +335,16 @@ hidden int __fmt(void *fn, void *arg, const char *format, va_list va) {
         break;
       }
       case 'c':
-        prec = 1;
-        flags |= FLAGS_PRECISION;
-        qchar = '\'';
-        p = charbuf;
-        charbuf[0] = va_arg(va, int);
-        goto FormatString;
+        if ((charbuf[0] = va_arg(va, int))) {
+          p = charbuf;
+          qchar = '\'';
+          flags |= FLAGS_PRECISION;
+          prec = 1;
+          goto FormatString;
+        } else {
+          __FMT_PUT('\0');
+          break;
+        }
       case 'm':
         p = weaken(strerror) ? weaken(strerror)(lasterr) : "?";
         signbit = 0;

test/libc/fmt/fmt_test.c

@@ -283,6 +283,15 @@ TEST(fmt, quoted) {
   ASSERT_STREQ("\"hello\"   ", gc(xasprintf("%-`*.*s", 10, 5, "hello")));
 }
 
+TEST(fmt, nulCharacter) {
+  char b[3] = {1, 1, 1};
+  ASSERT_EQ(1, snprintf(0, 0, "%c", 0));
+  ASSERT_EQ(1, snprintf(b, 3, "%c", 0));
+  ASSERT_EQ(0, b[0]);
+  ASSERT_EQ(0, b[1]);
+  ASSERT_EQ(1, b[2]);
+}
+
 TEST(fmt, regress) {
   char buf[512];
   const char *meth = "GET";