calling fwrite() with a null pointer causes undefined behavior. avoid it.

src/dump_atom.cpp

@@ -562,6 +562,7 @@ void DumpAtom::write_binary(int n, double *mybuf)
 
 void DumpAtom::write_string(int n, double *mybuf)
 {
+  if (mybuf)
     fwrite(mybuf,sizeof(char),n,fp);
 }
 

src/dump_cfg.cpp

@@ -233,6 +233,7 @@ void DumpCFG::write_data(int n, double *mybuf)
 
 void DumpCFG::write_string(int n, double *mybuf)
 {
+  if (mybuf)
     fwrite(mybuf,sizeof(char),n,fp);
 }
 

src/dump_custom.cpp

@@ -1234,6 +1234,7 @@ void DumpCustom::write_binary(int n, double *mybuf)
 
 void DumpCustom::write_string(int n, double *mybuf)
 {
+  if (mybuf)
     fwrite(mybuf,sizeof(char),n,fp);
 }
 

src/dump_local.cpp

@@ -399,6 +399,7 @@ void DumpLocal::write_data(int n, double *mybuf)
 
 void DumpLocal::write_string(int n, double *mybuf)
 {
+  if (mybuf)
     fwrite(mybuf,sizeof(char),n,fp);
 }
 

src/dump_xyz.cpp

@@ -194,6 +194,7 @@ void DumpXYZ::write_data(int n, double *mybuf)
 
 void DumpXYZ::write_string(int n, double *mybuf)
 {
+  if (mybuf)
     fwrite(mybuf,sizeof(char),n,fp);
 }