make Input::file(const char *) function safe for passing a null pointer

src/input.cpp

@@ -327,27 +327,28 @@ void Input::file(const char *filename)
   // call to file() will close filename and decrement nfile
 
   if (me == 0) {
-    if (nfile == maxfile)
-      error->one(FLERR,"Too many nested levels of input scripts");
+    if (nfile == maxfile) error->one(FLERR,"Too many nested levels of input scripts");
 
+    if (filename) {
       infile = fopen(filename,"r");
       if (infile == nullptr)
-      error->one(FLERR,"Cannot open input script {}: {}",
-                                   filename, utils::getsyserror());
-
+        error->one(FLERR,"Cannot open input script {}: {}", filename, utils::getsyserror());
       infiles[nfile++] = infile;
     }
+  }
 
   // process contents of file
 
   file();
 
   if (me == 0) {
+    if (filename) {
       fclose(infile);
       nfile--;
       infile = infiles[nfile-1];
     }
   }
+}
 
 /** Process a single command from a string in *single*
  *

src/library.cpp

@@ -532,8 +532,7 @@ void lammps_file(void *handle, const char *filename)
   BEGIN_CAPTURE
   {
     if (lmp->update->whichflag != 0)
-      lmp->error->all(FLERR,"Library error: issuing LAMMPS commands "
-                      "during a run is not allowed.");
+      lmp->error->all(FLERR, "Library error: issuing LAMMPS commands during a run is not allowed");
     else
       lmp->input->file(filename);
   }