api/helpers: implement C string to String helper

Use it in buffers.c

src/nvim/api/buffer.c

@@ -108,8 +108,8 @@ StringArray buffer_get_slice(Buffer buffer,
       goto end;
     }
 
-    rv.items[i].data = xstrdup((char *)ml_get_buf(buf, (linenr_T)lnum, false));
-    rv.items[i].size = strlen(rv.items[i].data);
+    const char *bufstr = (char *) ml_get_buf(buf, (linenr_T) lnum, false);
+    rv.items[i] = cstr_to_string(bufstr);
   }
 
 end:
@@ -307,9 +307,7 @@ String buffer_get_name(Buffer buffer, Error *err)
     return rv;
   }
 
-  rv.data = xstrdup((char *)buf->b_ffname);
-  rv.size = strlen(rv.data);
-  return rv;
+  return cstr_to_string((char *)buf->b_ffname);
 }
 
 void buffer_set_name(Buffer buffer, String name, Error *err)

src/nvim/api/helpers.c

@@ -339,6 +339,18 @@ tabpage_T * find_tab(Tabpage tabpage, Error *err)
   return rv;
 }
 
+String cstr_to_string(const char *str) {
+    if (str == NULL) {
+        return (String) { .data = NULL, .size = 0 };
+    }
+
+    size_t len = strlen(str);
+    return (String) {
+        .data = xmemdup(str, len),
+        .size = len
+    };
+}
+
 static bool object_to_vim(Object obj, typval_T *tv, Error *err)
 {
   tv->v_type = VAR_UNKNOWN;

src/nvim/api/helpers.h

@@ -86,5 +86,12 @@ win_T * find_window(Window window, Error *err);
 /// @return the tabpage pointer
 tabpage_T * find_tab(Tabpage tabpage, Error *err);
 
+/// Copies a C string into a String (binary safe string, characters + length)
+///
+/// @param str the C string to copy
+/// @return the resulting String, if the input string was NULL, then an
+///         empty String is returned
+String cstr_to_string(const char *str);
+
 #endif  // NVIM_API_HELPERS_H