mbedtls_mpi_sub_abs: Skip memcpy when redundant (#6701).

In some contexts, the output pointer may equal the first input
pointer, in which case copying is not only superfluous but results in
"Source and destination overlap in memcpy" errors from Valgrind (as I
observed in the context of ecp_double_jac) and a diagnostic message
from TrustInSoft Analyzer (as Pascal Cuoq reported in the context of
other ECP functions called by cert-app with a suitable certificate).

Signed-off-by: Aaron M. Ucko <ucko@ncbi.nlm.nih.gov>

ChangeLog.d/conditionalize-mbedtls_mpi_sub_abs-memcpy.txt

@@ -0,0 +1,4 @@
+Bugfix
+   * Fix potential undefined behavior in mbedtls_mpi_sub_abs().  Reported by
+     Pascal Cuoq using TrustInSoft Analyzer in #6701; observed independently by
+     Aaron Ucko under Valgrind.

library/bignum.c

@@ -1280,7 +1280,7 @@ int mbedtls_mpi_sub_abs(mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
     /* Set the high limbs of X to match A. Don't touch the lower limbs
      * because X might be aliased to B, and we must not overwrite the
      * significant digits of B. */
-    if (A->n > n) {
+    if (A->n > n && A != X) {
         memcpy(X->p + n, A->p + n, (A->n - n) * ciL);
     }
     if (X->n > A->n) {