plug-ins: fix #15812 PSD loader: heap-buffer-overflow ...

in fread_pascal_string

In plug-ins/file-psd/psd-util.c, the function fread_pascal_string()
allocates a buffer with g_malloc(len) and reads len bytes from the file
into it. The buffer is not null-terminated, but is assumed to be in
later code.
This causes it to read past the end of its allocated region with a
specially crafted PSD, causing a heap-buffer-overflow.

Fix this by alloocating one more byte than its length and set that
to '\0'.

plug-ins/file-psd/psd-util.c

@@ -274,7 +274,8 @@ fread_pascal_string (gint32        *bytes_read,
       return NULL;
     }
 
-  str = g_malloc (len);
+  str = g_malloc (len + 1);
+  str[len] = '\0';
   if (psd_read (input, str, len, error) < len)
     {
       psd_set_error (error);