Properly reallocate buffers when bit depth changes

This commit removes a suspicious assert and replaces it with a proper action
when a change in bit depth happens, fixing one of the assertion failures shown
by the test case in the following bug.

BUG=oss-fuzz:10061

There may still other assertion failures with this test case.

Change-Id: Ib9a24fca0f90ce032a5d7d4261e515dc01050211

aom_scale/generic/yv12config.c

diff --git a/aom_scale/generic/yv12config.c b/aom_scale/generic/yv12config.c
index 5cfba88..72cd28a 100644
--- a/aom_scale/generic/yv12config.c
+++ b/aom_scale/generic/yv12config.c
@@ -177,7 +177,11 @@
       ybf->y_buffer_8bit = (uint8_t *)aom_memalign(32, (size_t)yplane_size);
       if (!ybf->y_buffer_8bit) return -1;
     } else {
-      assert(!ybf->y_buffer_8bit);
+      if (ybf->y_buffer_8bit) {
+        aom_free(ybf->y_buffer_8bit);
+        ybf->y_buffer_8bit = NULL;
+        ybf->buf_8bit_valid = 0;
+      }
     }
 
     ybf->corrupted = 0; /* assume not corrupted by errors */