pem2der.c: fix double-free bug
pem2der.c: fix double-free bug

Found with Clang's `scan-build` tool.

load_file() allocates memory to a char** parameter. It then tries to fread() a
file, and if that fails, frees the memory and returns to caller. However, the
char** is not reset to NULL, which causes a double-free error when the caller
later passes it to free().

Mbed-TLS/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls