proper null termination of remark string

this might cause issues in other c libraries otherwise.
document and fix null termination

Author: kain88-de

package/MDAnalysis/lib/formats/include/readdcd.h

@@ -739,6 +739,9 @@ int write_dcdheader(fio_fd fd, const char *remarks, int N,
   fio_write_int32(fd, 3); /* the number of 80 character title strings */
 
   strncpy(title_string, remarks, 240);
+  // Enforce null-termination for long remark strings.
+  // Not a problem for MDAnalysis but maybe for other readers.
+  title_string[239] = '\0';
   WRITE(fd, title_string, 240);
 
   fio_write_int32(fd, 244);

package/MDAnalysis/lib/formats/libdcd.pyx

@@ -468,7 +468,8 @@ cdef class DCDFile:
         Parameters
         ----------
         remarks : str
-            remarks of DCD file. Shouldn't be more then 240 characters (ASCII)
+            remarks of DCD file. Writes up to 239 characters (ASCII). The
+            character 240 will be the null terminator
         natoms : int
             number of atoms to write
         istart : int
@@ -479,6 +480,7 @@ cdef class DCDFile:
             integrator time step. The time for 1 frame is nsavc * delta
         is_periodic : bool
             write unitcell information. Also pretends that file was written by CHARMM 24
+
         """
         if not self.is_open:
             raise IOError("No file open")

testsuite/MDAnalysisTests/formats/test_libdcd.py

@@ -318,7 +318,7 @@ def write_dcd(in_name, out_name, remarks='testing', header=None):
 
 @given(remarks=strategies.text(
     alphabet=string.printable, min_size=0,
-    max_size=240))  # handle the printable ASCII strings
+    max_size=239))  # handle the printable ASCII strings
 @example(remarks='')
 def test_written_remarks_property(remarks, tmpdir, dcd):
     # property based testing for writing of a wide range of string
@@ -327,7 +327,7 @@ def test_written_remarks_property(remarks, tmpdir, dcd):
     header = dcd.header
     header['remarks'] = remarks
     write_dcd(DCD, testfile, header=header)
-    expected_remarks = remarks[:240]
+    expected_remarks = remarks
     with DCDFile(testfile) as f:
         assert f.header['remarks'] == expected_remarks
 
@@ -340,6 +340,8 @@ def written_dcd(tmpdir_factory):
     testfile = str(testfile)
     write_dcd(DCD, testfile)
     Result = namedtuple("Result", "testfile, header, orgfile")
+    # throw away last char we didn't save due to null termination
+    header['remarks'] = header['remarks'][:-1]
     return Result(testfile, header, DCD)