Trim private_implementation__powers_of_10 array

Binary size, before:
30344 gen/lib/c/clang-9-dynamic/wuffs-base-floatconv.lo
30488 gen/lib/c/clang-9-static/wuffs-base-floatconv.o
31992 gen/lib/c/gcc-dynamic/wuffs-base-floatconv.lo
31992 gen/lib/c/gcc-static/wuffs-base-floatconv.o

After:
29512 gen/lib/c/clang-9-dynamic/wuffs-base-floatconv.lo
29656 gen/lib/c/clang-9-static/wuffs-base-floatconv.o
31160 gen/lib/c/gcc-dynamic/wuffs-base-floatconv.lo
31160 gen/lib/c/gcc-static/wuffs-base-floatconv.o
diff --git a/internal/cgen/base/floatconv-submodule-code.c b/internal/cgen/base/floatconv-submodule-code.c
index 5c5cc4d..e6de026 100644
--- a/internal/cgen/base/floatconv-submodule-code.c
+++ b/internal/cgen/base/floatconv-submodule-code.c
@@ -994,7 +994,7 @@
   // The look-up table was constructed so that it is already normalized: the
   // table entry's mantissa's MSB (most significant bit) is on.
   const uint32_t* po10 =
-      &wuffs_base__private_implementation__powers_of_10[5 * (exp10 + 326)];
+      &wuffs_base__private_implementation__powers_of_10[5 * (exp10 + 307)];
 
   // Normalize the man argument. The (man != 0) precondition means that a
   // non-zero bit exists.