Avoid passing out-of-bound pointers to 0-size memcpy

Doing so could be considered UB in a strict reading of the standard.
Avoid it.

contrib/lax_der_parsing.c

@@ -121,7 +121,7 @@ int ecdsa_signature_parse_der_lax(const secp256k1_context* ctx, secp256k1_ecdsa_
     /* Copy R value */
     if (rlen > 32) {
         overflow = 1;
-    } else {
+    } else if (rlen) {
         memcpy(tmpsig + 32 - rlen, input + rpos, rlen);
     }
 
@@ -133,7 +133,7 @@ int ecdsa_signature_parse_der_lax(const secp256k1_context* ctx, secp256k1_ecdsa_
     /* Copy S value */
     if (slen > 32) {
         overflow = 1;
-    } else {
+    } else if (slen) {
         memcpy(tmpsig + 64 - slen, input + spos, slen);
     }
 

contrib/lax_der_privatekey_parsing.c

@@ -45,7 +45,7 @@ int ec_privkey_import_der(const secp256k1_context* ctx, unsigned char *out32, co
     if (end < privkey+2 || privkey[0] != 0x04 || privkey[1] > 0x20 || end < privkey+2+privkey[1]) {
         return 0;
     }
-    memcpy(out32 + 32 - privkey[1], privkey + 2, privkey[1]);
+    if (privkey[1]) memcpy(out32 + 32 - privkey[1], privkey + 2, privkey[1]);
     if (!secp256k1_ec_seckey_verify(ctx, out32)) {
         memset(out32, 0, 32);
         return 0;

src/ecdsa_impl.h

@@ -140,7 +140,7 @@ static int secp256k1_der_parse_integer(secp256k1_scalar *r, const unsigned char
         overflow = 1;
     }
     if (!overflow) {
-        memcpy(ra + 32 - rlen, *sig, rlen);
+        if (rlen) memcpy(ra + 32 - rlen, *sig, rlen);
         secp256k1_scalar_set_b32(r, ra, &overflow);
     }
     if (overflow) {