[APFloat] Fix APFloat::getExactInverse when the input is nullptr. This is a regression introduced by r285105.

[hfinkel]

LGTM

[timshen]

Reproduced the bug and verified the fix with -DLLVM_USE_SANITIZER='Undefined'.

I'll submit this first to make the bot happy.

llvm/trunk/include/llvm/ADT/APFloat.h

public:
   }
 
   void toString(SmallVectorImpl<char> &Str, unsigned FormatPrecision = 0,
                 unsigned FormatMaxPadding = 3) const {
     return IEEE.toString(Str, FormatPrecision, FormatMaxPadding);
   }
 
   bool getExactInverse(APFloat *inv) const {
-    return IEEE.getExactInverse(&inv->IEEE);
+    return IEEE.getExactInverse(inv ? &inv->IEEE : nullptr);
   }
 
   friend hash_code hash_value(const APFloat &Arg);
   friend int ilogb(const APFloat &Arg) { return ilogb(Arg.IEEE); }
   friend APFloat scalbn(APFloat X, int Exp, roundingMode RM);
   friend APFloat frexp(const APFloat &X, int &Exp, roundingMode RM);
 };