Issue #1970 - Follow-up: Better fix for Unix that works on newer GCC.

My previous fix apparently only worked with GCC 7. Having that return at the
end doesn't seem to hurt anything on Windows, so I see no reason to ifdef it.

I don't remember where I heard this, but I vaguely remember hearing that
ending a function without a return statement may be undefined behavior that
differs between compilers and operating systems. If so, that would explain why
this has behaved so differently across platforms and compilers.

layout/generic/ReflowInput.cpp

@@ -2815,6 +2815,8 @@ nscoord ReflowInput::GetLineHeight() const {
                                mFrame->StyleContext(),
                                blockBSize,
                                nsLayoutUtils::FontSizeInflationFor(mFrame));
+
+  return mLineHeight;
 }
 
 void ReflowInput::SetLineHeight(nscoord aLineHeight) {