Mark two impossible code-paths with Q_UNREACHABLE

Change-Id: I8c04f512b078d4c13d759854b65f4d39b7b80e75
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>

src/corelib/text/qlocale.cpp

@@ -558,7 +558,7 @@ bool qt_splitLocaleName(QStringView name, QStringView *lang, QStringView *script
             state = NoState;
             break;
         case NoState: // Precluded by loop condition !
-            Q_ASSERT(!"QLocale: This should never happen");
+            Q_UNREACHABLE();
             break;
         }
     }

src/corelib/text/qlocale_tools.cpp

@@ -530,7 +530,8 @@ QString qulltoa(qulonglong number, int base, const QStringView zero)
 
             number /= base;
         }
-    } else {
+    } else { // zero should always be either a non-surrogate or a surrogate pair:
+        Q_UNREACHABLE();
         return QString();
     }