tool_easysrc.c: Test pointers against NULL

While validating a new Clang diagnostic (-Wnon-literal-null-conversion -
yes, the name isn't quite correct in this case, but it suffices) I found
a few violations of it in Curl.

src/tool_easysrc.c

@@ -140,7 +140,7 @@ CURLcode easysrc_perform(void)
     const char *c;
     CHKRET(easysrc_add(&easysrc_code, ""));
     /* Preamble comment */
-    for(i=0; ((c = srchard[i]) != '\0'); i++)
+    for(i=0; ((c = srchard[i]) != NULL); i++)
       CHKRET(easysrc_add(&easysrc_code, c));
     /* Each unconverted option */
     for(ptr=easysrc_toohard; ptr; ptr = ptr->next)
@@ -185,7 +185,7 @@ void dumpeasysrc(struct Configurable *config)
       int i;
       const char *c;
 
-      for(i=0; ((c = srchead[i]) != '\0'); i++)
+      for(i=0; ((c = srchead[i]) != NULL); i++)
         fprintf(out, "%s\n", c);
 
       /* Declare variables used for complex setopt values */
@@ -213,7 +213,7 @@ void dumpeasysrc(struct Configurable *config)
       for(ptr=easysrc_clean; ptr; ptr = ptr->next)
         fprintf(out, "  %s\n", ptr->data);
 
-      for(i=0; ((c = srcend[i]) != '\0'); i++)
+      for(i=0; ((c = srcend[i]) != NULL); i++)
         fprintf(out, "%s\n", c);
 
       if(fopened)