fix: read of uninitialized data at inChar[0]

This is a defensive fix, might cause unexpected program exit if triggered.
The loop will iterate at least once when nbytes==0, but this value
indicates XLookupString did not fill in any data, so the entire buffer
is undefined.

Using memset() before or inChar[0]=0 after XLookupString did not fix
the issue.  Using inChar[0]=0 immediately before the loop did fix as
well.  But this patch seems to be the best approach.

graphics/grTOGL1.c

@@ -599,7 +599,7 @@ keys_and_buttons:
 		idxmax = (nbytes == 0) ? 1 : nbytes;
 		for (idx = 0; idx < idxmax; idx++)
 		{
-		    if (inChar[idx] == 3)	/* Ctrl-C interrupt */
+		    if (nbytes > 0 && inChar[idx] == 3)	/* Ctrl-C interrupt */
 		    {
 			if (SigInterruptPending)
 			    MainExit(0);	/* double Ctrl-C */