prog: fix crash in blob mutation

If we deserialized a huge blob (larger than max blob size),
then we can get a negative size in the "Insert random bytes" case at:

		if r := int(maxLen) - len(data); n > r {
			n = r
		}

Don't insert bytes if data is already larger than maxLen.

prog/mutation.go

@@ -465,7 +465,7 @@ var mutateDataFuncs = [...]func(r *randGen, data []byte, minLen, maxLen uint64)
 	},
 	// Insert random bytes.
 	func(r *randGen, data []byte, minLen, maxLen uint64) ([]byte, bool) {
-		if len(data) == 0 {
+		if len(data) == 0 || uint64(len(data)) >= maxLen {
 			return data, false
 		}
 		n := r.Intn(16) + 1