Make %c parsing in *scanf() respect the C standard

Currently, `%c`-style directives always succeed even if there
are actually fewer characters in the input than requested.

Before the fix, the added test fails like this:

        ...
        EXPECT_EQ(2, sscanf("ab", "%c %c %c", &c2, &c3, &c4))
                need 2 (or 0x02 or '\2' or ENOENT) =
                 got 3 (or 0x03 or '\3' or ESRCH)
        ...
        EXPECT_EQ(0, sscanf("abcd", "%5c", s2))
                need 0 (or 0x0 or '\0') =
                 got 1 (or 0x01 or '\1' or EPERM)

musl and glibc pass this test.

libc/stdio/vcscanf.c

@@ -550,6 +550,11 @@ int __vcscanf(int callback(void *),    //
               if (!j && c == -1 && !items) {
                 items = -1;
                 goto Done;
+              } else if (rawmode && j != width) {
+                /* The C standard says that %c "matches a sequence of characters of
+                 * **exactly** the number specified by the field width". If we have
+                 * fewer characters, what we've just read is invalid. */
+                goto Done;
               } else if (!rawmode && j < bufsize) {
                 if (charbytes == sizeof(char)) {
                   buf[j] = '\0';

test/libc/stdio/sscanf_test.c

@@ -69,9 +69,17 @@ TEST(sscanf, testNonDirectiveCharacterMatching) {
 }
 
 TEST(sscanf, testCharacter) {
-  char c = 0;
-  EXPECT_EQ(1, sscanf("a", "%c", &c));
-  EXPECT_EQ('a', c);
+  char c1 = 0, c2 = c1, c3 = c2, c4 = c3;
+  char s1[32] = {0}, s2[32] = {0};
+  EXPECT_EQ(1, sscanf("a", "%c", &c1));
+  EXPECT_EQ(2, sscanf("ab", "%c %c %c", &c2, &c3, &c4));
+  EXPECT_EQ(1, sscanf("abcde", "%5c", s1));
+  EXPECT_EQ(0, sscanf("abcd", "%5c", s2));
+
+  EXPECT_EQ('a', c1);
+  EXPECT_EQ('a', c2);
+  EXPECT_EQ('b', c3);
+  EXPECT_STREQ("abcde", &s1[0]);
 }
 
 TEST(sscanf, testStringBuffer) {