proof: fix panic for keys with all 0xff bytes (#288)

Fixes #286. The `cpIncr()` function incorrectly incremented all bytes for values that are all `0xff` before appending `0x00`. In these cases it should keep the original value and append `0x00`.

CHANGELOG.md

@@ -10,6 +10,10 @@
 - [\#312](https://github.com/cosmos/iavl/pull/312) Added `MutableTree.SetInitialVersion()` to
   set the initial version after tree initialization.
 
+### Bug Fixes
+
+- [\#288](https://github.com/cosmos/iavl/pull/288) Fix panics when generating proofs for keys that are all `0xFF`.
+
 ## 0.14.0 (July 2, 2020)
 
 **Important information:** the pruning functionality introduced with IAVL 0.13.0 via the options 

util.go

@@ -72,6 +72,9 @@ func cpIncr(bz []byte) (ret []byte) {
 		}
 		ret[i] = byte(0x00)
 		if i == 0 {
+			// here, the original bz is all 0xFF, so we keep the original and append 0x00
+			// instead of returning all 0x00
+			ret = cp(bz)
 			return append(ret, 0x00)
 		}
 	}