fix sphinx VS non-reenterant malloc hang error

Index: src/searchd.cpp
===================================================================
--- src/searchd.cpp	(revision 2028)
+++ src/searchd.cpp	(working copy)
@@ -990,7 +990,7 @@
 void sigterm ( int )
 {
 	// in child, bail out immediately
-	if ( !g_bHeadDaemon && g_bAcceptUnlocked )
+	if ( !g_bHeadDaemon && g_bAcceptUnlocked && g_eWorkers != MPM_PREFORK )
 		exit ( 0 );
 
 	// in head or (possibly) locked preforked child, perform clean shutdown