Merge bitcoin#875: Avoid casting (void**) values.

2730618 Avoid casting (void**) values. Replaced with an expression that only casts (void*) values. (Russell O'Connor)

Pull request description:

ACKs for top commit:
  sipa:
    utACK 2730618
  real-or-random:
    utACK bitcoin-core/secp256k1@2730618
  jonasnick:
    utACK 2730618

Tree-SHA512: bdc1e9eefa10f79b744ef6ae83f379faff7bce9fb428c3bcfcc3f6e4e252e5c6543efbe0f84760709850948cbc8a432772c76a6c5f6b8cd18cb2d862b324912d

src/util.h

@@ -141,7 +141,7 @@ static SECP256K1_INLINE void *manual_alloc(void** prealloc_ptr, size_t alloc_siz
     VERIFY_CHECK(((unsigned char*)*prealloc_ptr - (unsigned char*)base) % ALIGNMENT == 0);
     VERIFY_CHECK((unsigned char*)*prealloc_ptr - (unsigned char*)base + aligned_alloc_size <= max_size);
     ret = *prealloc_ptr;
-    *((unsigned char**)prealloc_ptr) += aligned_alloc_size;
+    *prealloc_ptr = (unsigned char*)*prealloc_ptr + aligned_alloc_size;
     return ret;
 }