Cleanse 'bits' variable such that it doesn't get optimized away.

Fixes clang static analysis issue:

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An issue has been found in ./src/ecmult_gen_impl.h:150:5
Type:         Dead assignment
Description:  Value stored to 'bits' is never read

0: ./src/ecmult_gen_impl.h:153:5 - Value stored to 'bits' is never read
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Author: isle2983

src/ecmult_gen_impl.h

@@ -147,7 +147,7 @@ static void secp256k1_ecmult_gen(const secp256k1_ecmult_gen_context *ctx, secp25
         secp256k1_ge_from_storage(&add, &adds);
         secp256k1_gej_add_ge(r, r, &add);
     }
-    bits = 0;
+    SECP256K1_CLEANSE(bits);
     SECP256K1_CLEANSE(add);
     SECP256K1_CLEANSE(gnb);
 }