Fix maybe uninitialized variable warning

```
../runtime/float-builtins.cpp: In function ‘py::RawObject py::floatFromDigits(py::Thread*, const char*, word)’:
../runtime/float-builtins.cpp:119:14: error: ‘new_str’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
119 | delete[] new_str;
| ^~~~~~~
cc1plus: all warnings being treated as errors
```

runtime/float-builtins.cpp

···
 RawObject floatFromDigits(Thread* thread, const char* str, word length) {
   // TODO(T57022841): follow full CPython conversion for strings
   char* end;
-  char* new_str;
+  char* new_str = nullptr;
   const char* dup = str;
   word dup_length = length;
   bool release_memory = false;