Fix building with C++14, which errors out due to bool -> T* conversions
See also: https://bugs.gentoo.org/show_bug.cgi?id=592448

Patch by OpenSUSE

Index: src/effects/vamp/LoadVamp.cpp
===================================================================
--- src/effects/vamp/LoadVamp.cpp.orig
+++ src/effects/vamp/LoadVamp.cpp
@@ -266,7 +266,7 @@ Plugin *VampEffectsModule::FindPlugin(co
    Plugin *vp = PluginLoader::getInstance()->loadPlugin(key, 48000); // rate doesn't matter here
    if (!vp)
    {
-      return false;
+      return NULL;
    }
 
    // We limit the listed plugin outputs to those whose results can
Index: src/import/ImportFLAC.cpp
===================================================================
--- src/import/ImportFLAC.cpp.orig
+++ src/import/ImportFLAC.cpp
@@ -296,7 +296,7 @@ ImportFileHandle *FLACImportPlugin::Open
    int cnt;
    wxFile binaryFile;
    if (!binaryFile.Open(filename)) {
-      return false; // File not found
+      return NULL; // File not found
    }
 
 #ifdef USE_LIBID3TAG
@@ -313,7 +313,7 @@ ImportFileHandle *FLACImportPlugin::Open
 
    if (cnt == wxInvalidOffset || strncmp(buf, FLAC_HEADER, 4) != 0) {
       // File is not a FLAC file
-      return false;
+      return NULL;
    }
 
    // Open the file for import