Utils: Fix replace function in SmallString

You need to allocate the memory before you replace a smaller text with
a larger one. It has some overhead because you go two times over the text
but it is anyway not designed for a large text.

Change-Id: I2f56f1300a5daf9e26632b5b0667023a09ff4bd2
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

tests/unit/unittest/smallstring-test.cpp

@@ -753,6 +753,42 @@ TEST(SmallString, Clear)
     ASSERT_TRUE(text.isEmpty());
 }
 
+TEST(SmallString, NoOccurrencesForEmptyText)
+{
+    SmallString text;
+
+    auto occurrences = text.countOccurrence("text");
+
+    ASSERT_THAT(occurrences, 0);
+}
+
+TEST(SmallString, NoOccurrencesInText)
+{
+    SmallString text("here is some text, here is some text, here is some text");
+
+    auto occurrences = text.countOccurrence("texts");
+
+    ASSERT_THAT(occurrences, 0);
+}
+
+TEST(SmallString, SomeOccurrences)
+{
+    SmallString text("here is some text, here is some text, here is some text");
+
+    auto occurrences = text.countOccurrence("text");
+
+    ASSERT_THAT(occurrences, 3);
+}
+
+TEST(SmallString, SomeMoreOccurrences)
+{
+    SmallString text("texttexttext");
+
+    auto occurrences = text.countOccurrence("text");
+
+    ASSERT_THAT(occurrences, 3);
+}
+
 TEST(SmallString, ReplaceWithCharacter)
 {
     SmallString text("here is some text, here is some text, here is some text");