r.out.png: fix consecutive fclose calls on same pointer (OSGeo#4214)

This patch fixes two issues:

1. In one of the code paths, we are calling fclose on a
   file pointer which could potentially be NULL. Doing that
   would lead to undefined behavior. Check if a file pointer
   is NULL before closing it.

2. If we call fclose on same file pointer twice, in the
   second instance we could be closing file descriptor allocated
   to some other file, which typically happens to a freed
   descriptor.

This issue was found by using cppcheck tool.

Signed-off-by: Mohana Datta Yelugoti <ymdatta.work@gmail.com>

raster/r.out.png/main.c

@@ -63,7 +63,7 @@ int main(int argc, char *argv[])
     int png_compr, /* ret, */ do_alpha;
     struct Cell_head win;
     FILEDESC cellfile = 0;
-    FILE *fp;
+    FILE *fp = NULL;
 
     /* now goes from pnmtopng.c* -A.Sh */
     /*
@@ -207,20 +207,29 @@ int main(int argc, char *argv[])
         png_create_write_struct(PNG_LIBPNG_VER_STRING, &pnmtopng_jmpbuf_struct,
                                 pnmtopng_error_handler, NULL);
     if (png_ptr == NULL) {
-        fclose(fp);
+        if (fp) {
+            fclose(fp);
+            fp = NULL;
+        }
         G_fatal_error("cannot allocate LIBPNG structure");
     }
 
     info_ptr = png_create_info_struct(png_ptr);
     if (info_ptr == NULL) {
         png_destroy_write_struct(&png_ptr, (png_infopp)NULL);
-        fclose(fp);
+        if (fp) {
+            fclose(fp);
+            fp = NULL;
+        }
         G_fatal_error("cannot allocate LIBPNG structure");
     }
 
     if (setjmp(pnmtopng_jmpbuf_struct.jmpbuf)) {
         png_destroy_write_struct(&png_ptr, &info_ptr);
-        fclose(fp);
+        if (fp) {
+            fclose(fp);
+            fp = NULL;
+        }
         G_fatal_error("setjmp returns error condition (1)");
     }
 
@@ -360,7 +369,8 @@ int main(int argc, char *argv[])
     /* G_free (info_ptr); */
     png_destroy_write_struct(&png_ptr, &info_ptr); /* al 11/2000 */
 
-    fclose(fp);
+    if (fp)
+        fclose(fp);
 
     if (wld_flag->answer) {
         if (do_stdout)