Fix warnings found with -Wsign-compare.

diff --git a/dlls/msi/string.c b/dlls/msi/string.c
index 59affef..8cc4dc1 100644
--- a/dlls/msi/string.c
+++ b/dlls/msi/string.c
@@ -52,9 +52,9 @@
     msistring *strings; /* an array of strings (in the tree) */
 };
 
-static int msistring_makehash( const WCHAR *str )
+static UINT msistring_makehash( const WCHAR *str )
 {
-    int hash = 0;
+    UINT hash = 0;
 
     if (str==NULL)
         return hash;
@@ -106,7 +106,7 @@
 
 static int st_find_free_entry( string_table *st )
 {
-    int i, sz;
+    UINT i, sz;
     msistring *p;
 
     TRACE("%p\n", st);
@@ -135,7 +135,7 @@
     return st->freeslot;
 }
 
-static void st_mark_entry_used( string_table *st, int n )
+static void st_mark_entry_used( string_table *st, UINT n )
 {
     if( n >= st->maxcount )
         return;
@@ -351,7 +351,7 @@
  */
 UINT msi_string2idW( string_table *st, LPCWSTR str, UINT *id )
 {
-    int hash;
+    UINT hash;
     UINT i, r = ERROR_INVALID_PARAMETER;
 
     hash = msistring_makehash( str );