Don't segfault on null bytes in REPTs and MACROs

Previously, the copyrept() and copymacro() functions would halt their
first loop (in which they determine the length of the block)
prematurely,  causing an underflow when setting len, eventually causing
memory issues.

Whilst this doesn’t solve the len underflow entirely (e.g. if the file
ends immediately without an ENDR/ENDM), it should help with this exact
scenario of null bytes (as #50).

src/asm/asmy.y

@@ -142,8 +142,9 @@ void	copyrept( void )
 {
 	SLONG	level=1, len, instring=0;
 	char	*src=pCurrentBuffer->pBuffer;
+	char	*bufferEnd = pCurrentBuffer->pBufferStart + pCurrentBuffer->nBufferSize;
 
-	while( *src && level )
+	while( src < bufferEnd && level )
 	{
 		if( instring==0 )
 		{
@@ -217,8 +218,9 @@ void	copymacro( void )
 {
 	SLONG	level=1, len, instring=0;
 	char	*src=pCurrentBuffer->pBuffer;
+	char	*bufferEnd = pCurrentBuffer->pBufferStart + pCurrentBuffer->nBufferSize;
 
-	while( *src && level )
+	while( src < bufferEnd && level )
 	{
 		if( instring==0 )
 		{