mapstring: optimize msStringToUpper(), msStringToLower()

Traverse the string only once.  Also, this removes code which
triggered -Wsign-compare.

Author: MaxKellermann

mapstring.c

@@ -377,25 +377,17 @@ char *msIntToString(int value)
 
 void msStringToUpper(char *string)
 {
-  int i;
-
   if (string != NULL) {
-    for (i = 0; i < strlen(string); i++) {
-      string[i] = toupper(string[i]);
-    }
-    return;
+    for (; *string; ++string)
+      *string = toupper(*string);
   }
 }
 
 void msStringToLower(char *string)
 {
-  int i;
-
   if (string != NULL) {
-    for (i = 0; i < strlen(string); i++) {
-      string[i] = tolower(string[i]);
-    }
-    return;
+    for (; *string; ++string)
+      *string = tolower(*string);
   }
 }