Bug 1468131 - Avoid a ubsan complaint in GetTrimmableWhitespaceCount r=emilio

ubsan was complaining about the expression
```
    const char* str = aFrag->Get1b() + aStartOffset;
```
when `aFrag->Get1b() == nullptr` and `aStartOffset == -1`, because the addition generates an invalid pointer.

Due to other logic in the function, we would never dereference that pointer, so it was reasonably harmless, but this patch silences the complaint.

Differential Revision: https://phabricator.services.mozilla.com/D49345

--HG--
extra : moz-landing-system : lando

layout/generic/nsTextFrame.cpp

@@ -832,6 +832,10 @@ static uint32_t GetTrimmableWhitespaceCount(const nsTextFragment* aFrag,
                                             int32_t aStartOffset,
                                             int32_t aLength,
                                             int32_t aDirection) {
+  if (!aLength) {
+    return 0;
+  }
+
   int32_t count = 0;
   if (aFrag->Is2b()) {
     const char16_t* str = aFrag->Get2b() + aStartOffset;