Add checking of size param and clarify usage in doc

doc/dox_comments/header_files/srp.h

@@ -340,6 +340,8 @@ int wc_SrpSetPrivate(Srp* srp, const byte* priv, word32 size);
     This function MUST be called after wc_SrpSetPassword or wc_SrpSetVerifier.
     The function wc_SrpSetPrivate may be called before wc_SrpGetPublic.
 
+    Caller must observe value of size upon return to know the actual size.
+
     \return 0 Success
     \return BAD_FUNC_ARG Returned if srp, pub, or size is null.
     \return SRP_CALL_ORDER_E Returned if wc_SrpGetPublic is called out
@@ -349,8 +351,8 @@ int wc_SrpSetPrivate(Srp* srp, const byte* priv, word32 size);
 
     \param srp the Srp structure.
     \param pub the buffer to write the public ephemeral value.
-    \param size the the buffer size in bytes. Will be updated with
-    the ephemeral value size.
+    \param size IN: the buffer size in bytes.
+                OUT: Will be updated with the ephemeral value size.
 
     _Example_
     \code
@@ -369,7 +371,7 @@ int wc_SrpSetPrivate(Srp* srp, const byte* priv, word32 size);
     wc_SrpSetPassword(&srp, password, passwordSize)
 
     byte public[64];
-    word32 publicSz = 0;
+    word32 publicSz = sizeof(public);
 
     if( wc_SrpGetPublic(&srp, public, &publicSz) != 0)
     {

wolfcrypt/src/srp.c

@@ -627,8 +627,10 @@ int wc_SrpGetPublic(Srp* srp, byte* pub, word32* size)
         }
     }
 
+    /* Clear buffer */
+    XMEMSET(pub, 0, *size);
+
     /* extract public key to buffer */
-    XMEMSET(pub, 0, modulusSz);
     if (!r) r = mp_to_unsigned_bin(pubkey, pub);
     if (!r) *size = (word32)mp_unsigned_bin_size(pubkey);