CryptoPkg/BaseCryptLib: Fix mismatched memory allocation/free

The malloc/free (instead of AllocatePool/FreePool) were used directly
in some wrapper implementations, which was designed to leverage the
light-weight memory management routines at Runtime phase.
The malloc/free and AllocatePool/FreePool usages are required to be
matched, after extra memory size info header was introduced in malloc
wrapper.

This patch corrects two memory allocation cases, which requires the
caller to free the buffer with FreePool() outside the function call.

And some comments were also added to clarify the correct memory
release functions if it's the caller's responsibility to free the
memory buffer.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ting Ye <ting.ye@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Qin Long <qin.long@intel.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>

CryptoPkg/Library/BaseCryptLib/Pk/CryptPkcs7SignNull.c

@@ -33,7 +33,8 @@ WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
   @param[in]  OtherCerts       Pointer to an optional additional set of certificates to
                                include in the PKCS#7 signedData (e.g. any intermediate
                                CAs in the chain).
-  @param[out] SignedData       Pointer to output PKCS#7 signedData.
+  @param[out] SignedData       Pointer to output PKCS#7 signedData. It's caller's
+                               responsibility to free the buffer with FreePool().
   @param[out] SignedDataSize   Size of SignedData in bytes.
 
   @retval FALSE  This interface is not supported.