av1_first_pass: fix non-zero offset of null ptr

check num_planes before incrementing x->plane[].src.buf

this fixes four non-zero offset of a null pointer warnings under clang
-fsanitize=undefined:
firstpass.c:947: runtime error: applying non-zero offset 8 to null
pointer
firstpass.c:948: runtime error: applying non-zero offset 8 to null
pointer
firstpass.c:955: runtime error: applying non-zero offset to non-null
pointer 0x0000000000b0 produced null pointer
firstpass.c:957: runtime error: applying non-zero offset to non-null
pointer 0x0000000000b0 produced null pointer

Analogous to https://aomedia-review.googlesource.com/c/aom/+/162443.

Change-Id: I7cf8f8b3dcd6442d2e192de98333eed9d5b34d6d

av1/encoder/firstpass.c

diff --git a/av1/encoder/firstpass.c b/av1/encoder/firstpass.c
index 113c068..4ec7997 100644
--- a/av1/encoder/firstpass.c
+++ b/av1/encoder/firstpass.c
@@ -944,18 +944,22 @@
 
       // Adjust to the next column of MBs.
       x->plane[0].src.buf += 16;
-      x->plane[1].src.buf += uv_mb_height;
-      x->plane[2].src.buf += uv_mb_height;
+      if (num_planes > 1) {
+        x->plane[1].src.buf += uv_mb_height;
+        x->plane[2].src.buf += uv_mb_height;
+      }
 
       recon_yoffset += 16;
       recon_uvoffset += uv_mb_height;
     }
     // Adjust to the next row of MBs.
     x->plane[0].src.buf += 16 * x->plane[0].src.stride - 16 * cm->mb_cols;
-    x->plane[1].src.buf +=
-        uv_mb_height * x->plane[1].src.stride - uv_mb_height * cm->mb_cols;
-    x->plane[2].src.buf +=
-        uv_mb_height * x->plane[1].src.stride - uv_mb_height * cm->mb_cols;
+    if (num_planes > 1) {
+      x->plane[1].src.buf +=
+          uv_mb_height * x->plane[1].src.stride - uv_mb_height * cm->mb_cols;
+      x->plane[2].src.buf +=
+          uv_mb_height * x->plane[1].src.stride - uv_mb_height * cm->mb_cols;
+    }
 
     aom_clear_system_state();
   }