s/t/bamtools_filter.cpp: simplify GetScriptContents().

[vpa1977]

This MR resolves https://github.com/pezmaster31/bamtools/issues/237

Changes:

• Make 'feof(inFile)' a while condition (to avoid infinite loop in the code)
• break when fgets() returns NULL (no data to read)
• check ferror() - fgets() return value is undefined behaviour in this case.

Testing:

• Running bamtools filter -script filter_script -in ./tests/data/sam_spec_example.bam -out out.bam with filter_script

  {
  "filters" : 
  [
    { "id" : "inAnyErrorReadGroup",
      "tag" : "RG:ERR*"
    },
    { "id" : "highMapQuality",
      "mapQuality" : ">=75"
    },
    { "id" : "bothMatesMapped",
      "isMapped" : "true",
      "isMateMapped" : "true"
    }
  ],
  "rule" : "!inAnyErrorReadGroup & (highMapQuality | bothMatesMapped)"
  }

  succeeds

[SoapZA]

@vpa1977 I've improved your commits a bit. Could you try HEAD once more before I cut a new release?