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 ),