Revert "vty: Fix bad use of vector_slot()"

The patch seemed sensible, but introduces a segfault when hitting tab
on the interactive VTY. Reproduction example:

  osmo-msc
  telnet 127.0.0.1 4254
  OsmoMSC> enable <TAB>

So we need to understand what that line of code actually intends to do.
Until then, revert this to avoid the segfault.

The segfault happens at:

  Program received signal SIGSEGV, Segmentation fault.
  0x00007ffff7bc0894 in cmd_complete_command_real (vline=0x5555558d59e0, vty=0x5555558d57b0, status=0x7fffffffe024) at ../../../../src/libosmocore/src/vty/command.c:1953
  1953                if (*(char *)vector_slot(vline, index) == '\0')

This reverts commit e9e9e427b78271941a25a63567fc2ec2bb9e4433.

Change-Id: I3fe213bdfb96de9469aae64e67000dafee59302e
diff --git a/src/vty/command.c b/src/vty/command.c
index 355fb5d..21b26b4 100644
--- a/src/vty/command.c
+++ b/src/vty/command.c
@@ -1950,7 +1950,7 @@
 
 		/* In case of 'command \t' pattern.  Do you need '?' command at
 		   the end of the line. */
-		if (*(char *)vector_slot(vline, index) == '\0')
+		if (vector_slot(vline, index) == '\0')
 			*status = CMD_ERR_NOTHING_TODO;
 		else
 			*status = CMD_ERR_NO_MATCH;