Fix static analyzer issues caught by clang

system/core/libsparse/sparse_read.c:260:51: warning: Dereference of null pointer (loaded from variable 'crc_ptr')
                        ret = process_crc32_chunk(fd, chunk_data_size, *crc_ptr);
system/core/libsparse/sparse_read.c:404:9: warning: Potential leak of memory pointed to by 'buf'
        return 0;

Fixes leak in sparse_file_read_normal, and null pointer dereference
crash if an image with an obsolete CRC chunk was read with CRC checking
disabled.

Bug: 26904425

Change-Id: Ibc72cd37602929ae2c248bea1cdd1d22ea03baaf