aya-bpf: helpers::read_{kernel,user}_str_bytes additional len check

Without additional check verifier fails swearing len lower bound is a
negative value. Observed on v5.19/ubuntu 22.04.

Signed-off-by: Tomasz Jonak <tomasz@deepfence.io>

bpf/aya-bpf/src/helpers.rs

@@ -430,6 +430,11 @@ pub unsafe fn bpf_probe_read_user_str_bytes(
         return Err(-1);
     }
 
+    // verifier complains if this check is missing on v5.19
+    if len < 1 {
+        return Err(-1);
+    }
+
     // len includes NULL byte
     Ok(&dest[..len - 1])
 }
@@ -583,6 +588,11 @@ pub unsafe fn bpf_probe_read_kernel_str_bytes(
         return Err(-1);
     }
 
+    // verifier complains if this check is missing on v5.19
+    if len < 1 {
+        return Err(-1);
+    }
+
     // len includes NULL byte
     Ok(&dest[..len - 1])
 }