img_alloc_helper: make align var unsigned

quiets an integer sanitizer warning:
vpx/src/vpx_image.c:101:25: runtime error: implicit conversion from
type 'int' of value -2 (32-bit, signed) to type 'unsigned int' changed
the value to 4294967294 (32-bit, unsigned)

Change-Id: Ifeac31cc80811081c1ba10aadaa94dc36cd46efa
diff --git a/vpx/src/vpx_image.c b/vpx/src/vpx_image.c
index 2b7411f..2a7afc0 100644
--- a/vpx/src/vpx_image.c
+++ b/vpx/src/vpx_image.c
@@ -22,7 +22,7 @@
                                      unsigned char *img_data) {
   unsigned int h, w, s, xcs, ycs, bps;
   unsigned int stride_in_bytes;
-  int align;
+  unsigned int align;
 
   if (img != NULL) memset(img, 0, sizeof(vpx_image_t));