Fix allocation from stack bug.

For large numbers of SNPs allocating from the stack caused a segfault.
This fixes the issue by allocating from the heap with malloc.

src/snp_sites.c

@@ -85,7 +85,7 @@ int generate_snp_sites(char filename[], int exclude_gaps, char suffix[])
  internal_nodes[a] = 0;
  }
 
- char* bases_for_snps[number_of_snps];
+ char** bases_for_snps = malloc(number_of_snps * sizeof(char *));
 
  for(i = 0; i < number_of_snps; i++)
  {