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

@@ -830,11 +830,13 @@ static void ReadCallVarAss(TypSymbolSet follow, Char mode)
         }
         else {
             Match( S_ASSIGN, ":=", follow );
+            UInt currLHSGVar = STATE(CurrLHSGVar);
             if ( LEN_PLIST(STATE(StackNams)) == 0 || !STATE(IntrCoding) ) {
                 STATE(CurrLHSGVar) = (ref.type == R_GVAR ? ref.var : 0);
             }
             ReadExpr( follow, 'r' );
             AssignRef(ref);
+            STATE(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 );
+                          ^