Fix Extern Byref declarations to not require initializer

It should be possible to declare extern references, and like any other
externs, they are not initialized at the Extern, but at the corresponding
Dim (if any).
http://www.freebasic.net/forum/viewtopic.php?p=211604#p211604

Author: dkl

src/compiler/parser-decl-var.bas

@@ -1710,8 +1710,8 @@ function cVarDecl _
 			else
 				'' default initialization
 				if( sym ) then
-					'' Byref? Always requires an explicit initializer
-					if( symbIsRef( sym ) ) then
+					'' Allocating a Byref variable? Always requires an explicit initializer
+					if( (not symbIsExtern( sym )) and symbIsRef( sym ) ) then
 						errReport( FB_ERRMSG_MISSINGREFINIT )
 						hSkipStmt( )
 						exit function

tests/dim/byref-common.bi

@@ -0,0 +1,11 @@
+namespace fbc_tests.dim_.byref_
+namespace externs
+	extern i as integer
+	extern byref ri as integer
+	type SomeUDT
+		dummy as integer
+		static i as integer
+		static byref ri as integer
+	end type
+end namespace
+end namespace

tests/dim/byref.bas

@@ -1,4 +1,5 @@
 #include "fbcu.bi"
+#include "byref-common.bi"
 
 namespace fbc_tests.dim_.byref_
 
@@ -583,13 +584,25 @@ namespace privateDtor
 	end sub
 end namespace
 
+namespace externs
+	sub test cdecl( )
+		CU_ASSERT( @ri = @i )
+		CU_ASSERT( @SomeUDT.ri = @SomeUDT.i )
+		CU_ASSERT( i  = &hEE112233 )
+		CU_ASSERT( ri = &hEE112233 )
+		CU_ASSERT( SomeUDT.i  = &hEE445566 )
+		CU_ASSERT( SomeUDT.ri = &hEE445566 )
+	end sub
+end namespace
+
 private sub ctor( ) constructor
 	fbcu.add_suite( "tests/dim/byref" )
 	fbcu.add_test( "simpleVars", @simpleVars.test )
 	fbcu.add_test( "allDtypes", @allDtypes.test )
 	fbcu.add_test( "callByrefFunctionPtrThroughByref", @callByrefFunctionPtrThroughByref.test )
 	fbcu.add_test( "noCtorsCalled", @noCtorsCalled.test )
 	fbcu.add_test( "privateDtor", @privateDtor.test )
+	fbcu.add_test( "externs", @externs.test )
 end sub
 
 end namespace

tests/dim/byref2.bas

@@ -0,0 +1,11 @@
+#include "fbcu.bi"
+#include "byref-common.bi"
+
+namespace fbc_tests.dim_.byref_
+namespace externs
+	dim shared i as integer = &hEE112233
+	dim shared byref ri as integer = i
+	dim shared SomeUDT.i as integer = &hEE445566
+	dim shared byref SomeUDT.ri as integer = SomeUDT.i
+end namespace
+end namespace