perf scripts python: exported-sql-viewer.py: Fix error when shrinking / enlarging font

Fix the following error if shrink / enlarge font is used with the help
window.

  Traceback (most recent call last):
    File "tools/perf/scripts/python/exported-sql-viewer.py", line 2791, in ShrinkFont
      ShrinkFont(win.view)
  AttributeError: 'HelpWindow' object has no attribute 'view'

Committer testing:

Before, matches above output:

  $ python ~acme/libexec/perf-core/scripts/python/exported-sql-viewer.py ~/c/adrian.hunter/simple-retpoline.db
  Traceback (most recent call last):
    File "/home/acme/libexec/perf-core/scripts/python/exported-sql-viewer.py", line 2780, in EnlargeFont
      EnlargeFont(win.view)
  AttributeError: 'HelpWindow' object has no attribute 'view'
  $

After:

No more tracebacks, but the fonts don't get enlarged, which is kinda
frustrating...
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20190503120828.25326-2-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/scripts/python/exported-sql-viewer.py

@@ -2755,6 +2755,14 @@ class MainWindow(QMainWindow):
 		help_menu = menu.addMenu("&Help")
 		help_menu.addAction(CreateAction("&Exported SQL Viewer Help", "Helpful information", self.Help, self, QKeySequence.HelpContents))
 
+	def Try(self, fn):
+		win = self.mdi_area.activeSubWindow()
+		if win:
+			try:
+				fn(win.view)
+			except:
+				pass
+
 	def Find(self):
 		win = self.mdi_area.activeSubWindow()
 		if win:
@@ -2772,12 +2780,10 @@ class MainWindow(QMainWindow):
 				pass
 
 	def ShrinkFont(self):
-		win = self.mdi_area.activeSubWindow()
-		ShrinkFont(win.view)
+		self.Try(ShrinkFont)
 
 	def EnlargeFont(self):
-		win = self.mdi_area.activeSubWindow()
-		EnlargeFont(win.view)
+		self.Try(EnlargeFont)
 
 	def EventMenu(self, events, reports_menu):
 		branches_events = 0