| Bugfix | |
| * Fix a null pointer dereference when mbedtls_mpi_exp_mod() was called with | |
| A=0 represented with 0 limbs. This bug could not be triggered by code | |
| that constructed A with one of the mbedtls_mpi_read_xxx functions | |
| (including in particular TLS code) since those always built an mpi object | |
| with at least one limb. Credit to OSS-Fuzz. Fixes #4641. |