byte-based printf family: emit a nul byte for %lc with argument zero

this is contrary to the spec as written, which requires %lc to behave
as if it were %ls on a 2-wchar_t buffer containing the argument and
zero. however, apparently no other implementations conform to the spec
as written, and in response to Austin Group issue #1647, WG14 chose to
align with existing practice and have %lc produce output for this case.

src/stdio/vfprintf.c

@@ -588,6 +588,7 @@ static int printf_core(FILE *f, const char *fmt, va_list *ap, union arg *nl_arg,
 			}
 			p = MAX(p, z-a + !arg.i);
 			break;
+		narrow_c:
 		case 'c':
 			*(a=z-(p=1))=arg.i;
 			fl &= ~ZERO_PAD;
@@ -602,6 +603,7 @@ static int printf_core(FILE *f, const char *fmt, va_list *ap, union arg *nl_arg,
 			fl &= ~ZERO_PAD;
 			break;
 		case 'C':
+			if (!arg.i) goto narrow_c;
 			wc[0] = arg.i;
 			wc[1] = 0;
 			arg.p = wc;