fix: check for existing name on branch creation

Author: kroggen

sqlite3.c

@@ -63204,6 +63204,9 @@ SQLITE_PRIVATE int pragma_new_branch(sqlite3 *db, int iDb, char *zBranchName, ch
     if( (rc=sqlite3BranchCheckReloadDb(pPager,0))!=SQLITE_OK ) return rc;
   }
 
+  /* check if the new branch name already exists */
+  if( sqlite3BranchFind(lmdb,zBranchName) > 0 ) return SQLITE_MISUSE;
+
   /* check if the source branch is valid */
   source_branch_id = sqlite3BranchFind(lmdb, zSourceBranch);
   if( source_branch_id <= 0 ) return SQLITE_NOTFOUND;