Test for underflow before subtraction.

Found with -Wtype-limits.

Change-Id: I41cdbb7e6564b715dfe445877a89594371fdeef0
Signed-off-by: Piotr Sikora <piotrsikora@google.com>
Reviewed-on: https://boringssl-review.googlesource.com/6462
Reviewed-by: Adam Langley <agl@google.com>

crypto/bio/pair.c

diff --git a/crypto/bio/pair.c b/crypto/bio/pair.c
index 6f78890..fba4be2 100644
--- a/crypto/bio/pair.c
+++ b/crypto/bio/pair.c
@@ -256,8 +256,8 @@
     return 0;
   }
 
+  assert(peer_b->len >= bytes_read);
   peer_b->len -= bytes_read;
-  assert(peer_b->len >= 0);
   assert(peer_b->offset + bytes_read <= peer_b->size);
 
   /* Move read offset. If zero_copy_write_lock == 1 we must advance the