Issue #2978 - Avoid empty replacements in macro expansion.

If the replacements array is empty, don't attempt to do any work on the
existing macro, and return. This should prevent infinite recursion in
some cases.
Order matters here: we're relying on the `if` statement shortcut so
`collectMacroArgs()` doesn't get called.

gfx/angle/src/compiler/preprocessor/MacroExpander.cpp

@@ -220,7 +220,8 @@ bool MacroExpander::expandMacro(const Macro &macro,
         assert(macro.type == Macro::kTypeFunc);
         std::vector<MacroArg> args;
         args.reserve(macro.parameters.size());
-        if (!collectMacroArgs(macro, identifier, &args, &replacementLocation))
+        if (replacements->size() == 0 ||
+            !collectMacroArgs(macro, identifier, &args, &replacementLocation))
             return false;
 
         replaceMacroParams(macro, args, replacements);