Refuse to load "literals" that can be confused with registers

The 's' operand overloads the tags for pids and ports to represent
X and Y registers, respectively.  At load time, refuse to load the
module if the "literal" term is not a pid or port, as it would be
interpreted as a register.

This does not happen with normally compiled code, but it can happen
if the compiler (or beam_asm) is abused like in the following example:

make_bad() ->
    Pid = self(),
    Forms = [{attribute, 0, module, bad_s_operand},
             {attribute, 0, export, [{test, 0}]},
             {function, 0, test, 0,
              [{clause, 0, [], [],
                [{call,0,{atom,0,tuple_size},[{integer, 0, Pid}]}]}]}],
    {ok, Module, Bin} = compile:forms(Forms, [no_copt,no_postopt,report_errors]),
    code:load_binary(Module, "bad_s_operand.erl", Bin).

With this commit applied, the following message will be printed
when make_bad() is run:

=ERROR REPORT==== 10-Nov-2017::14:47:59 ===
Loading of bad_s_operand.erl failed: badfile

=ERROR REPORT==== 10-Nov-2017::14:47:59 ===
beam/beam_load.c(2396): Error loading function bad_s_operand:test/0: op bif1_body: bsd:
  the term '<0.60.0>' would be confused with a register

1 files changed, 12 insertions, 2 deletions

diff --git a/erts/emulator/beam/beam_load.c b/erts/emulator/beam/beam_load.c
index 7331c331a6..4fcfea527c 100644
--- a/erts/emulator/beam/beam_load.c
+++ b/erts/emulator/beam/beam_load.c
@@ -2384,8 +2384,18 @@ load_code(LoaderState* stp)
 		    code[ci++] = NIL;
 		    break;
 		case TAG_q:
-		    new_literal_patch(stp, ci);
-		    code[ci++] = tmp_op->a[arg].val;
+                    {
+                        BeamInstr val = tmp_op->a[arg].val;
+                        Eterm term = stp->literals[val].term;
+                        new_literal_patch(stp, ci);
+                        code[ci++] = val;
+                        switch (loader_tag(term)) {
+                        case LOADER_X_REG:
+                        case LOADER_Y_REG:
+                            LoadError1(stp, "the term '%T' would be confused "
+                                       "with a register", term);
+                        }
+                    }
 		    break;
 		default:
 		    LoadError1(stp, "bad tag %d for general source",