memory from getenv is not ours to free

regrssion from...

commit c3074cb5
Author: Chris Sherlock <chris.sherlock79@gmail.com>
Date:   Sun Jan 7 00:39:33 2018 +1100

    vcl: remove dead code

Change-Id: I3295ece62e97d00e56db95d0005d13d2a97984c2
Reviewed-on: https://gerrit.libreoffice.org/53858Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/unx/gtk3/gtk3gtkframe.cxx

@@ -385,8 +385,7 @@ bool GtkSalFrame::doKeyCallback( guint state,
     vcl::DeletionListener aDel( this );
 
 #if OSL_DEBUG_LEVEL > 0
-    char* pKeyDebug = nullptr;
-    pKeyDebug = getenv("VCL_GTK3_PAINTDEBUG");
+    const char* pKeyDebug = getenv("VCL_GTK3_PAINTDEBUG");
 
     if (pKeyDebug && *pKeyDebug == '1')
     {
@@ -413,8 +412,6 @@ bool GtkSalFrame::doKeyCallback( guint state,
             }
         }
     }
-
-    free(pKeyDebug);
 #endif
 
     /*