winedbg: fix DEBUG_ReadLine

[Andrew de Quincey]

Was passing a NULL buffer (i.e. not initially HeapAlloced()) to a call which 
then called HeapReAlloc() on it. Result: not good.

Example of erroneous behaviour: winedbg something and do "break MessageBoxA". 
Anything entered into the prompt as to which to choose will cause the 
debugger to crash.
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