stratum: do not abort the daemon on a malformed 63-character job_id

The "EWBF 31 bytes job_id fix" in stratum_mining_submit completes a
63-character job_id with each hex digit in turn and feeds the result
straight to uint256(). ParseHex() stops at the first non-hex character
and returns a shorter vector without signalling an error, and
base_blob(const std::vector<unsigned char>&) asserts vch.size() == 32.

So a single mining.submit whose job_id is any 63-character string
containing a non-hex byte -- 63 spaces will do -- aborts the node.
asserts are live in release builds here: -DNDEBUG appears only in
leveldb's own makefile, never in configure.ac, and the shipped binary
still carries the assertion string. The dispatch loop catches UniValue
and std::exception; abort() goes through both.

Size-check each candidate before constructing, as ParseUInt256() a few
hundred lines up already does for the ordinary path. If none of the 16
completions parse, ret stays null, misses work_templates, and the
handler returns false exactly as it does for any unknown job.

Not gated on IsHex(job_id_str): 63 is odd and IsHex() requires an even
length, so that test would disable the EWBF path this code exists for.

src/stratum.cpp

@@ -1334,7 +1334,16 @@ UniValue stratum_mining_submit(StratumClient& client, const UniValue& params)
         if (job_id_str.length() == 63) {
             fEWBFJobIDFixNeeded = true;
             for(const auto& hexDigit : hexDigits) {
-                ret = uint256(ParseHex(job_id_str + hexDigit));
+                // ParseHex() stops at the first non-hex character and returns a SHORT vector
+                // without signalling an error, and base_blob(const std::vector<unsigned char>&)
+                // asserts vch.size() == 32. Constructing without checking therefore lets any
+                // 63-character job_id containing a non-hex byte abort the daemon -- from an
+                // unauthenticated client, before any other validation. Skip bad candidates
+                // instead; if none of the 16 completions parse, ret stays null, misses
+                // work_templates below, and the handler returns false cleanly.
+                std::vector<unsigned char> vch = ParseHex(job_id_str + hexDigit);
+                if (vch.size() != 32) continue;
+                ret = uint256(vch);
                 if (work_templates.count(ret)) break;
             }
         }