[MC] Return early when .fill size is negative

[phosek]

Summary

Rather than invoking emitFill with negative size, which may trigger
an undefined behavior, return immediately after emitting the warning.

[phosek]

This triggers an error on ubsan build because of an invalid shift in MCStreamer::emitFill because of the negative Size. This is not a bug; even though Expr has an incorrect value, it's never used because NumValues is set to 0, but we want to avoid further ubsan failures.

lib/MC/MCParser/AsmParser.cpp

 
   if (FillSize < 0) {
     Warning(SizeLoc, "'.fill' directive with negative size has no effect");
-    NumValues = MCConstantExpr::create(0, getStreamer().getContext());
+    return false;
   }
   if (FillSize > 8) {
     Warning(SizeLoc, "'.fill' directive with size greater than 8 has been truncated to 8");