md5: remove unreachable len operand in wc_Md5Update

Premise:  md5.c:345 `if (md5 == NULL || (data == NULL && len > 0))
          return BAD_FUNC_ARG;` establishes !(data == NULL && len > 0), i.e.
          data != NULL || len == 0.
Claim:    md5.c:361 `if (data == NULL && len == 0)` -- the `len == 0` operand.
Proof:    under the left conjunct data == NULL, the premise forces len == 0.
          len is word32, so !(len > 0) is exactly len == 0 -- no signed
          wraparound case to consider. Neither data nor len is written between
          :345 and :361. The operand is invariantly true when evaluated and its
          independence pair is unreachable.
Scope:    the intervening code is the
          WOLFSSL_ASYNC_CRYPT && WC_ASYNC_ENABLE_MD5 block (:349-355), which
          may return early but writes neither variable, and the buffLen range
          check (:358). The argument holds in all four combinations of those
          two macros.
Evidence: llvm-cov MC/DC records this condition's pair as uncovered
          (reports/md5/GAPS.md row 361:9:361:33:1); it is the sole residual
          holding md5.c at 12/13.

Proof check: gcc does not fold this (it does not propagate the
correlation between the entry guard's two operands across the branch), so the
implication was machine-checked instead: with data == NULL held true, all 2^32
word32 values of len were enumerated and every one that passes the :345 guard
has len == 0. Zero counterexamples -- exhaustive, not sampled.

wolfcrypt/src/md5.c

@@ -358,8 +358,8 @@ int wc_Md5Update(wc_Md5* md5, const byte* data, word32 len)
     if (md5->buffLen >= WC_MD5_BLOCK_SIZE)
         return BUFFER_E;
 
-    if (data == NULL && len == 0) {
-        /* valid, but do nothing */
+    if (data == NULL) {
+        /* len is 0 here: valid, but do nothing */
         return 0;
     }