Fix sbi_console_getchar return value if no UART is present

The UART drivers all return -1 if no character is present, and so that's
what we should do if there's no UART at all.

See discussion on https://github.com/riscv-non-isa/riscv-sbi-doc/issues/82

machine/mtrap.c

@@ -77,7 +77,7 @@ static uintptr_t mcall_console_getchar()
   } else if (htif) {
     return htif_console_getchar();
   } else {
-    return '\0';
+    return (uintptr_t)-1;
   }
 }