opt: Don't segfault if a string's default is NULL

Instead show '(nil)', like other people do. This is distinguishable
from a similar looking string value, because the latter is shown with
double quotes while NULL's nil has no quotes.
Signed-off-by: Douglas Bagnall <douglas@halo.gen.nz>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

ccan/opt/helpers.c

@@ -202,6 +202,7 @@ void opt_show_invbool(char buf[OPT_SHOW_LEN], const bool *b)
 
 void opt_show_charp(char buf[OPT_SHOW_LEN], char *const *p)
 {
+	if (*p){
 		size_t len = strlen(*p);
 		buf[0] = '"';
 		if (len > OPT_SHOW_LEN - 2)
@@ -210,6 +211,10 @@ void opt_show_charp(char buf[OPT_SHOW_LEN], char *const *p)
 		buf[1+len] = '"';
 		if (len < OPT_SHOW_LEN - 2)
 			buf[2+len] = '\0';
+	}
+	else {
+		strncpy(buf, "(nil)", OPT_SHOW_LEN);
+	}
 }
 
 /* Show an integer value, various forms. */