Fix size of cached powers of 5 array.

We need the powers of 5 up to 5**512 because we only jump straight to
underflow when the exponent is less than -512 (or larger than 308).

Author: colesbury

Include/internal/pycore_dtoa.h

@@ -36,7 +36,7 @@ struct _dtoa_state {
 #define Bigint_Kmax 7
 
 /* The size of the cached powers of 5 array */
-#define Bigint_Pow5max 7
+#define Bigint_Pow5max 8
 
 #ifndef PRIVATE_MEM
 #define PRIVATE_MEM 2304

Python/dtoa.c

@@ -2807,7 +2807,7 @@ _PyDtoa_Init(PyInterpreterState *interp)
     }
     p5s[0] = p5;
 
-    // compute 5**8, 5**16, 5**32, ..., 5**256
+    // compute 5**8, 5**16, 5**32, ..., 5**512
     for (Py_ssize_t i = 1; i < Bigint_Pow5max; i++) {
         p5 = mult(p5, p5);
         if (p5 == NULL) {