Make programming check more forceful

Coverity complains that a g_return_if_fail() that checks on
parse_value() respecting its API might leak. Be more forceful, and
assert() instead. Can't leak if you crash *taps temple*.

shared-mime-info-2.1/src/update-mime-database.c:1482: alloc_arg: "parse_value" allocates memory that is stored into "parsed_mask".
shared-mime-info-2.1/src/update-mime-database.c:1488: leaked_storage: Variable "parsed_mask" going out of scope leaks the storage it points to.
1486|   	{
1487|   		g_string_free(parsed_value, TRUE);
1488|-> 		g_return_if_fail(parsed_mask == NULL);
1489|   	}
1490|   	else
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