fileio: fix free buffer and off-by-one in readline

Author: nalgeon

src/fileio/scan.c

@@ -60,15 +60,15 @@ static ssize_t readline(char** lineptr, size_t* n, FILE* stream) {
     }
     p = bufptr;
     while (c != EOF) {
-        if ((ssize_t)(p - bufptr)+1 > (ssize_t)(size)) {
-            size_t offset = p - bufptr; // save offset
-	    size = size + 128;
-	    char * new_buf = realloc(bufptr, size);
+        if ((size_t)(p - bufptr) >= size - 1) {
+            size = size + 128;
+            char* new_buf = realloc(bufptr, size);
             if (new_buf == NULL) {
+                free(bufptr);
                 return -1;
             }
-	    bufptr = new_buf;
-	    p = bufptr + offset; // recalculate p using the saved offset
+            p = new_buf + (p - bufptr);
+            bufptr = new_buf;
         }
         *p++ = c;
         if (c == '\n') {