Reading user input requires a foreground layer

Fixes crash from commands such as setenv, readreg, 'layout select' etc.
when starting with 'screen -md'. Detailed report at #24152, debian bug
 #496750.

src/input.c

@@ -120,6 +120,9 @@ int data;
   int maxlen;
   struct inpdata *inpdata;
   
+  if (!flayer)
+    return;
+
   if (len > 100)
     len = 100;
   if (!(mode & INP_NOECHO))