WebDAV: fix cppcheck warning about uninitialized memory read

cppcheck cannot know that one of the two if checks must succeed
and warns about the case when both are false. This shouldn't
happen, but it doesn't hurt to initialize the const char * pointer
to a default value.

src/backends/webdav/WebDAVSource.cpp

@@ -194,7 +194,7 @@ void ContextSettings::lookupAuthProvider()
     InitStateString password;
 
     // prefer source config if anything is set there
-    const char *credentialsFrom;
+    const char *credentialsFrom = "undefined";
     if (m_sourceConfig) {
         identity = m_sourceConfig->getUser();
         password = m_sourceConfig->getPassword();