VirKeyboard: Fixed possible segfault

Fix to VirKeyboard, so invalid keys do not get registered.
Based upon recommendation by zenon48 in:
http://www.kvraudio.com/forum/viewtopic.php?t=266432
(cherry picked from commit b9e7cb07bd00780967c68d17deab2190e18a604d)

plugins/zynaddsubfx/ChangeLog

@@ -960,3 +960,6 @@
 
 28 Oct 2009 (Paul Nasca)
 	    - Disable "bw" control on Reverb when Bandwidth mode is not enabled
+
+18 Nov 2009 (Mark McCurry)
+	    - Fixed segfault in VirKeyBoard

plugins/zynaddsubfx/src/UI/VirKeyboard.cc

@@ -103,7 +103,7 @@ if ((event==FL_PUSH)||(event==FL_DRAG)||(event==FL_RELEASE)){
        };
    };
 
-   if (((event==FL_PUSH)||(event==FL_DRAG))&&
+   if ((keypos!=-1)&&((event==FL_PUSH)||(event==FL_DRAG))&&
        (Fl::event_shift()==0)) {
         presskey(kpos,1,1);
    };

plugins/zynaddsubfx/src/UI/VirKeyboard.fl

@@ -138,7 +138,7 @@ if ((event==FL_PUSH)||(event==FL_DRAG)||(event==FL_RELEASE)){
        };
    };
 
-   if (((event==FL_PUSH)||(event==FL_DRAG))&&
+   if ((keypos!=-1)&&((event==FL_PUSH)||(event==FL_DRAG))&&
        (Fl::event_shift()==0)) {
         presskey(kpos,1,1);
    };