Fix PKCS#7/CMS EnvelopedData decrypt of definite-length constructed [0]

wc_PKCS7_DecodeEnvelopedData() failed to decrypt a valid CMS/SCEP
EnvelopedData whose encryptedContent is a definite-length constructed
context [0] wrapping a single definite-length OCTET STRING
(A0 82 xx xx  04 82 yy yy  <ciphertext>). This is the encoding emitted
by Go's crypto/pkcs7 stacks (e.g. micromdm/scep), so SCEP enrollment
against those servers failed at the CertRep decrypt step.

The decoder recognized the constructed [0] tag but its handler assumed
the BER indefinite-length fragmented form: a run of OCTET STRINGs
terminated by an EOC (00 00). The definite-length single-OCTET-STRING
form has no EOC, so after decrypting the one OCTET STRING the loop kept
scanning for a terminator that never comes. Streaming builds returned
WC_PKCS7_WANT_READ_E; NO_PKCS7_STREAM builds spun. The primitive
(80 ...) and constructed-indefinite (A0 80 ... 00 00) encodings were
already handled.

Right after reading the [0] length, detect the case where the [0]
definite length is filled exactly by a single inner OCTET STRING, skip
that inner header, and fall into the existing primitive single-shot
decrypt path. The check runs before the NO_PKCS7_STREAM bounds check so
both streaming and non-streaming builds are corrected, and the inner
length is read with NO_USER_CHECK because the ciphertext may not be
fully buffered yet in streaming mode; the size-equality test validates
the structure instead. That equality is computed in word32 to avoid
signed-overflow undefined behavior on a crafted oversized inner length.
The unwrap only fires for the constructed-definite single-OCTET-STRING
shape; primitive and indefinite encodings are untouched.

Add test_wc_PKCS7_DecodeEnvelopedData_constructedDefiniteOctet
(tests/api/test_pkcs7.c). wolfSSL's encoder never emits this form, so
the test transcodes a normal encode into it. It covers the positive
case plus three negatives that pin the unwrap guard: two OCTET STRINGs,
a [0] longer than the inner OCTET STRING, and a non-OCTET-STRING inner.

Author: Frauschi