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().

programs/util/pem2der.c

@@ -134,6 +134,7 @@ static int load_file( const char *path, unsigned char **buf, size_t *n )
     {
         fclose( f );
         free( *buf );
+        *buf = NULL;
         return( -1 );
     }