[MC] writeFragment - assert MCFragment::FT_Fill length is legal.

Silence (clang/MSVC) static analyzer warnings that the fragment data may either write out of bounds of the local array or reference uninitialized data.

llvm/lib/MC/MCAssembler.cpp

@@ -572,6 +572,7 @@ static void writeFragment(raw_ostream &OS, const MCAssembler &Asm,
     unsigned VSize = FF.getValueSize();
     const unsigned MaxChunkSize = 16;
     char Data[MaxChunkSize];
+    assert(0 < VSize && VSize <= MaxChunkSize && "Illegal fragment fill size");
     // Duplicate V into Data as byte vector to reduce number of
     // writes done. As such, do endian conversion here.
     for (unsigned I = 0; I != VSize; ++I) {