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

src/dump_atom.cpp

@@ -562,7 +562,8 @@ void DumpAtom::write_binary(int n, double *mybuf)
 
 void DumpAtom::write_string(int n, double *mybuf)
 {
-  fwrite(mybuf,sizeof(char),n,fp);
+  if (mybuf)
+    fwrite(mybuf,sizeof(char),n,fp);
 }
 
 /* ---------------------------------------------------------------------- */

src/dump_cfg.cpp

@@ -233,7 +233,8 @@ void DumpCFG::write_data(int n, double *mybuf)
 
 void DumpCFG::write_string(int n, double *mybuf)
 {
-  fwrite(mybuf,sizeof(char),n,fp);
+  if (mybuf)
+    fwrite(mybuf,sizeof(char),n,fp);
 }
 
 /* ---------------------------------------------------------------------- */

src/dump_custom.cpp

@@ -1234,7 +1234,8 @@ void DumpCustom::write_binary(int n, double *mybuf)
 
 void DumpCustom::write_string(int n, double *mybuf)
 {
-  fwrite(mybuf,sizeof(char),n,fp);
+  if (mybuf)
+    fwrite(mybuf,sizeof(char),n,fp);
 }
 
 /* ---------------------------------------------------------------------- */

src/dump_local.cpp

@@ -399,7 +399,8 @@ void DumpLocal::write_data(int n, double *mybuf)
 
 void DumpLocal::write_string(int n, double *mybuf)
 {
-  fwrite(mybuf,sizeof(char),n,fp);
+  if (mybuf)
+    fwrite(mybuf,sizeof(char),n,fp);
 }
 
 /* ---------------------------------------------------------------------- */

src/dump_xyz.cpp

@@ -194,7 +194,8 @@ void DumpXYZ::write_data(int n, double *mybuf)
 
 void DumpXYZ::write_string(int n, double *mybuf)
 {
-  fwrite(mybuf,sizeof(char),n,fp);
+  if (mybuf)
+    fwrite(mybuf,sizeof(char),n,fp);
 }
 
 /* ---------------------------------------------------------------------- */