Support randombytes() with size == 0, but don't force implementations to support it

src/libsodium/randombytes/randombytes.c

@@ -46,7 +46,9 @@ randombytes_uniform(const uint32_t upper_bound)
 void
 randombytes_buf(void * const buf, const size_t size)
 {
-    implementation->buf(buf, size);
+    if (size > (size_t) 0U) {
+        implementation->buf(buf, size);
+    }
 }
 
 int

src/libsodium/randombytes/salsa20/randombytes_salsa20_random.c

@@ -85,6 +85,7 @@ safe_read(const int fd, void * const buf_, size_t count)
     unsigned char *buf = (unsigned char *) buf_;
     ssize_t        readnb;
 
+    assert(count > (size_t) 0U);
     do {
         while ((readnb = read(fd, buf, count)) < (ssize_t) 0 &&
                errno == EINTR);

src/libsodium/randombytes/sysrandom/randombytes_sysrandom.c

@@ -45,6 +45,7 @@ safe_read(const int fd, void * const buf_, size_t count)
     unsigned char *buf = (unsigned char *) buf_;
     ssize_t        readnb;
 
+    assert(count > (size_t) 0U);
     do {
         while ((readnb = read(fd, buf, count)) < (ssize_t) 0 &&
                errno == EINTR);