crypto: Add missing WriteBE16 function

Also add fuzz test, mimicing the WriteLE16 one.

Author: laanwj

src/crypto/common.h

@@ -70,6 +70,12 @@ uint64_t static inline ReadBE64(const unsigned char* ptr)
     return be64toh_internal(x);
 }
 
+void static inline WriteBE16(unsigned char* ptr, uint16_t x)
+{
+    uint16_t v = htobe16_internal(x);
+    memcpy(ptr, &v, 2);
+}
+
 void static inline WriteBE32(unsigned char* ptr, uint32_t x)
 {
     uint32_t v = htobe32_internal(x);

src/test/fuzz/crypto_common.cpp

@@ -35,6 +35,10 @@ FUZZ_TARGET(crypto_common)
     WriteLE64(writele64_arr.data(), random_u64);
     assert(ReadLE64(writele64_arr.data()) == random_u64);
 
+    std::array<uint8_t, 2> writebe16_arr;
+    WriteBE16(writebe16_arr.data(), random_u16);
+    assert(ReadBE16(writebe16_arr.data()) == random_u16);
+
     std::array<uint8_t, 4> writebe32_arr;
     WriteBE32(writebe32_arr.data(), random_u32);
     assert(ReadBE32(writebe32_arr.data()) == random_u32);