Fix DownEnv / UpEnv handling

Also get rid of STATE(ErrorLVars0)

This is an example for DownEnv/UpEnv usage:

gap> f:=lvl -> 1/lvl + f(lvl-1);
function( lvl ) ... end
gap> f(3);
Error, Rational operations: <divisor> must not be zero in
  return 1 / lvl + f( (lvl - 1) ); called from
f( lvl - 1 ) called from
f( lvl - 1 ) called from
f( lvl - 1 ) called from
<function "f">( <arguments> )
 called from read-eval loop at line 12 of *stdin*
you can replace <divisor> via 'return <divisor>;'
brk> lvl;
0
brk> UpEnv(1); lvl;
0
brk> DownEnv(1); lvl;
1
brk> DownEnv(1); lvl;
2
brk> UpEnv(1); lvl;
1
brk> DownEnv(1); lvl;
2
brk> DownEnv(1); lvl;
3
brk> DownEnv(1); lvl;
3
brk> UpEnv(1); lvl;
2

Note that before this commit, the very last UpEnv(1) incorrectly returned
us to level 0.

src/gap.c

@@ -950,7 +950,7 @@ void DownEnvInner( Int depth )
       depth = 0;
     }
     /* ... then go back to the top, and later go down to the appropriate level. */
-    STATE(ErrorLVars) = STATE(ErrorLVars0);
+    STATE(ErrorLVars) = STATE(BaseShellContext);
     STATE(ErrorLLevel) = 0;
     STATE(ShellContext) = STATE(BaseShellContext);
   }

src/gap.h

@@ -201,9 +201,6 @@ extern void ErrorReturnVoid (
             Int                 arg2,
             const Char *        msg2 );
 
-/* TL: extern Obj ErrorLVars;
-   TL: extern Obj ErrorLVars0;
- */
 
 /****************************************************************************
 **

src/gapstate.h

@@ -141,9 +141,8 @@ typedef struct GAPState {
     UInt UserHasQUIT;
     Obj  ShellContext;
     Obj  BaseShellContext;
-    Obj  ErrorLVars0;       // the initial ErrorLVars value, i.e. for the lvars were the break occurred
     Obj  ErrorLVars;        // ErrorLVars as modified by DownEnv / UpEnv
-    Int  ErrorLLevel;       // record where on the stack ErrorLVars is relative to the top, i.e. ErrorLVars0
+    Int  ErrorLLevel;       // record where on the stack ErrorLVars is relative to the top, i.e. BaseShellContext
 
     /* From objects.c */
     Obj PrintObjThis;

src/read.c

@@ -2755,7 +2755,6 @@ ExecStatus ReadEvalCommand ( Obj context, UInt *dualSemicolon )
     volatile UInt                readTilde;
     volatile UInt                currLHSGVar;
     volatile Obj                 errorLVars;
-    volatile Obj                 errorLVars0;
     syJmp_buf           readJmpError;
 #ifdef HPCGAP
     int                 lockSP;
@@ -2792,9 +2791,7 @@ ExecStatus ReadEvalCommand ( Obj context, UInt *dualSemicolon )
     STATE(CurrLHSGVar) = 0;
     RecreateStackNams(context);
     errorLVars = STATE(ErrorLVars);
-    errorLVars0 = STATE(ErrorLVars0);
     STATE(ErrorLVars) = context;
-    STATE(ErrorLVars0) = STATE(ErrorLVars);
 #ifdef HPCGAP
     lockSP = RegionLockSP();
 #endif
@@ -2862,7 +2859,6 @@ ExecStatus ReadEvalCommand ( Obj context, UInt *dualSemicolon )
     STATE(ReadTilde)   = readTilde;
     STATE(CurrLHSGVar) = currLHSGVar;
     STATE(ErrorLVars) = errorLVars;
-    STATE(ErrorLVars0) = errorLVars0;
 
     /* copy the result (if any)                                            */
     STATE(ReadEvalResult) = STATE(IntrResult);