MINOR: sample: Validate the number of bits for the sha2 converter

Instead of failing the conversion when an invalid number of bits is
given the sha2 converter now fails with an appropriate error message
during startup.

The sha2 converter was introduced in d4376302377e4f51f43a183c2c91d929b27e1ae3,
which is in 2.1 and higher.
diff --git a/reg-tests/converter/sha2.vtc b/reg-tests/converter/sha2.vtc
index 0354b0a..6ca021a 100644
--- a/reg-tests/converter/sha2.vtc
+++ b/reg-tests/converter/sha2.vtc
@@ -8,7 +8,7 @@
 server s1 {
 	rxreq
 	txresp
-} -repeat 3 -start
+} -repeat 2 -start
 
 haproxy h1 -conf {
     defaults
@@ -28,7 +28,6 @@
 	http-response set-header SHA2-256   "%[var(txn.hash),sha2(256),hex,lower]"
 	http-response set-header SHA2-384   "%[var(txn.hash),sha2(384),hex,lower]"
 	http-response set-header SHA2-512   "%[var(txn.hash),sha2(512),hex,lower]"
-	http-response set-header SHA2-invalid   "%[var(txn.hash),sha2(1),hex,lower]"
 
 	default_backend be
 
@@ -46,7 +45,6 @@
 	expect resp.http.sha2-256 == "6b86b273ff34fce19d6b804eff5a3f5747ada4eaa22f1d49c01e52ddb7875b4b"
 	expect resp.http.sha2-384 == "47f05d367b0c32e438fb63e6cf4a5f35c2aa2f90dc7543f8a41a0f95ce8a40a313ab5cf36134a2068c4c969cb50db776"
 	expect resp.http.sha2-512 == "4dff4ea340f0a823f15d3f4f01ab62eae0e5da579ccb851f8db9dfe84c58b2b37b89903a740e1ee172da793a6e79d560e5f7f9bd058a12a280433ed6fa46510a"
-	expect resp.http.sha2-invalid == ""
 	txreq -url "/" \
 	  -hdr "Hash: 2"
 	rxresp
@@ -56,5 +54,4 @@
 	expect resp.http.sha2-256 == "d4735e3a265e16eee03f59718b9b5d03019c07d8b6c51f90da3a666eec13ab35"
 	expect resp.http.sha2-384 == "d063457705d66d6f016e4cdd747db3af8d70ebfd36badd63de6c8ca4a9d8bfb5d874e7fbd750aa804dcaddae7eeef51e"
 	expect resp.http.sha2-512 == "40b244112641dd78dd4f93b6c9190dd46e0099194d5a44257b7efad6ef9ff4683da1eda0244448cb343aa688f5d3efd7314dafe580ac0bcbf115aeca9e8dc114"
-	expect resp.http.sha2-invalid == ""
 } -run