Fix single-read bug from refactoring
Bug fix if first read only occurs once

$ samtools view -F 4 -h ERR019322_vs_DQ631913_X2.bam | head -n 5
@SQ	   SN:gi|108885496|gb|DQ631913.1|	     LN:15120
@PG	   ID:bwa				     PN:bwa	VN:0.7.10-r789	CL:...
ERR019322.295 121 gi|108885496|gb|DQ631913.1| 4798 0 103M = 4798 0 CAAGAGAGAGATAACTAGGGTGAATACTACGACTTAGATGTACTGTTTAAGTTTCTACGGGCTACAGAGAAGCTACCCGCAGGGTATATATTTGCTCATTACA ...
ERR019322.24    0 gi|108885496|gb|DQ631913.1| 9246 0 48M  *    0 0 TATTTTGCTGTTTTCTTTAATAGTCTTGAGACTAAGGGACTTCTAGAG ...
ERR019322.24  256 gi|108885496|gb|DQ631913.1| 1686 0 48M  *    0 0 * ...

$ samtools view -F 4 -h ERR019322_vs_DQ631913_X2.bam | ~/repositories/picobio/sambam/sam_undouble_circles.py -c ../refs/DQ631913.fas
Lengths of 1 circular references loaded
@SQ	SN:gi|108885496|gb|DQ631913.1|	LN:7560
@PG	ID:bwa				PN:bwa	VN:0.7.10-r789	CL:/home/pc40583/Downloads/bwa-0.7.10/bwa mem -p -S -a ../refs/DQ631913_X2.fas ../bloom_filter/ERR019322_P.fastq.bgz
ERR019322.295 121 gi|108885496|gb|DQ631913.1| 4798 0 103M = 4798 0 CAAGAGAGAGATAACTAGGGTGAATACTACGACTTAGATGTACTGTTTAAGTTTCTACGGGCTACAGAGAAGCTACCCGCAGGGTATATATTTGCTCATTACA ..
ERR019322.24    0 gi|108885496|gb|DQ631913.1| 1686 0 48M  *    0 0 TATTTTGCTGTTTTCTTTAATAGTCTTGAGACTAAGGGACTTCTAGAG ..

Traceback (most recent call last):
  File "/home/pc40583/repositories/picobio/sambam/sam_undouble_circles.py", line 333, in <module>
    for line in dedup_batch(undouble_circle_mappings(restore_seq(batch))):
  File "/home/pc40583/repositories/picobio/sambam/sam_undouble_circles.py", line 265, in dedup_batch
    for line in sam_lines:
  File "/home/pc40583/repositories/picobio/sambam/sam_undouble_circles.py", line 221, in undouble_circle_mappings
    for line in sam_lines:
  File "/home/pc40583/repositories/picobio/sambam/sam_undouble_circles.py", line 195, in restore_seq
    if qname == last_name and get_frag(flag) == last_frag:
UnboundLocalError: local variable 'last_name' referenced before assignment