Use the virtual real-mode interrupt table if low DOS memory isn't
mapped.

diff --git a/msdos/interrupts.c b/msdos/interrupts.c
index d065401..c8662e5 100644
--- a/msdos/interrupts.c
+++ b/msdos/interrupts.c
@@ -64,7 +64,7 @@
  */
 FARPROC16 INT_GetRMHandler( BYTE intnum )
 {
-    return ((FARPROC16*)DOSMEM_MemoryBase())[intnum];
+    return ((FARPROC16*)DOSMEM_SystemBase())[intnum];
 }
 
 
@@ -77,7 +77,7 @@
 {
     TRACE("Set real mode interrupt vector %02x <- %04x:%04x\n",
                  intnum, HIWORD(handler), LOWORD(handler) );
-    ((FARPROC16*)DOSMEM_MemoryBase())[intnum] = handler;
+    ((FARPROC16*)DOSMEM_SystemBase())[intnum] = handler;
 }