Fix missing syntax warning for using undefined gvar

We normally issue a warning when code is parsed that uses a variable
identifier which has not yet been defined, which GAP then resolves as the
name of a gvar.

However, we suppress this warning if it is for the assignment to a global
variable.

But in some rare cases, this suppressed too much: if the very next line after
such an assignment wasn't an assignment itself, and accessed the gvar, we did
not print a warning.

src/read.c

@@ -806,11 +806,13 @@ static void ReadCallVarAss(TypSymbolSet follow, Char mode)
  }
  else {
  Match( S_ASSIGN, ":=", follow );
+ UInt currLHSGVar = ReaderState()->CurrLHSGVar;
  if ( LEN_PLIST(STATE(StackNams)) == 0 || !STATE(IntrCoding) ) {
  ReaderState()->CurrLHSGVar = (ref.type == R_GVAR ? ref.var : 0);
  }
  ReadExpr( follow, 'r' );
  AssignRef(ref);
+ ReaderState()->CurrLHSGVar = currLHSGVar;
  }
  }
 

tst/testbugfix/2018-06-29-CurrLHSGVar.tst

@@ -0,0 +1,19 @@
+# This always produced a warning"
+gap> Unbind(string);
+gap> for i in [ 1 .. 10 ] do
+> string := "aaaabbbb";
+> xxx := List( [], x -> string );
+> od;
+Syntax warning: Unbound global variable in stream:3
+ xxx := List( [], x -> string );
+ ^
+
+# ... but this did not; now it does
+gap> Unbind(string);
+gap> for i in [ 1 .. 10 ] do
+> string := "aaaabbbb";
+> List( [], x -> string );
+> od;
+Syntax warning: Unbound global variable in stream:3
+ List( [], x -> string );
+ ^

tst/teststandard/ctblsymm.tst

@@ -12,6 +12,7 @@ gap> c3:= CharacterTable( CyclicGroup( 3 ) );;
 gap> n:= 3;;
 gap> wr:= CharacterTableWreathSymmetric( c3, n );;
 gap> irr:= Irr( wr );;
+gap> betas:=fail;;
 gap> for i in [ 1 .. Length( irr ) ] do
 > betas:= List( CharacterParameters( wr )[i], BetaSet );
 > if List( ClassParameters( wr ),