fbc: fix bad error recovery on BYREF intializer

For example:
    Type UDT Extends Object
        Declare Constructor(Byval nop As Integer = 0)
    End Type
    Constructor UDT(Byval nop As Integer = 0)
    End Constructor

    Dim Byref As UDT u = UDT()

changelog.txt

@@ -130,6 +130,7 @@ Version 1.10.0
 - fbc: extend lifetime of temporary variables in WITH TYPE(...) expressions to the END WITH statement
 - fbc: #cmdline "-r -rr -R -RR -o objfile ..." needs to restart the parser and handle changes in keeping the ASM file.  In the case of '-o objfile' a major restart is needed since some initialization of filenames is done before parsing starts.  By default the temporary ASM file was being kept even if #cmdline options indicate to keep it.
 - sf.net #569: gfxlib2: GetMouse/SetMouse scaling problem in QB modes Screen 2 & 8 - adjust GetMouse/SetMouse by internal scanline_size
+- fbc: fbc: fix bad error recovery on BYREF intializer.  'Dim Byref As UDT u = UDT()' would case compiler crash if UDT constructor argument optional
 
 
 Version 1.09.0

src/compiler/ast-node-arg.bas

@@ -1029,6 +1029,12 @@ function astNewARG _
 	'' optional/default?
 	if( arg = NULL ) then
 		arg = hCreateOptArg( param )
+
+		'' still NULL? then hCreateOptArg() failed
+		if( arg = NULL ) then
+			function = NULL
+			exit function
+		end if
 	end if
 
 	if( dtype = FB_DATATYPE_INVALID ) then
@@ -1049,6 +1055,7 @@ function astNewARG _
 			else
 				errReportParam( parent->sym, parent->call.args+1, NULL, FB_ERRMSG_ILLEGALASSIGNMENT )
 			end if
+			function = NULL
 			exit function
 		end if
 	end if