Upgrade mtools to 3.0.37

This version contains a fix for -Wstring-concatenation error.

Test: presubmit
Bug: 175068488
Change-Id: Id4f2ae49da91a481ab2625fdf17668689b640794
diff --git a/streamcache.c b/streamcache.c
index 4277ef0..be50248 100644
--- a/streamcache.c
+++ b/streamcache.c
@@ -54,17 +54,17 @@
 	atexit(finish_sc);
 }
 
-Stream_t *open_root_dir(unsigned char drive, int flags, int *isRop)
+Stream_t *open_root_dir(char drive, int flags, int *isRop)
 {
 	Stream_t *Fs;
 
 	init_streamcache();
 
-	drive = toupper(drive);
-	
+	drive = (char)toupper(drive);
+
 	/* open the drive */
-	if(fss[drive])
-		Fs = fss[drive];
+	if(fss[(unsigned char)drive])
+		Fs = fss[(unsigned char)drive];
 	else {
 		Fs = fs_init(drive, flags, isRop);
 		if (!Fs){
@@ -72,7 +72,7 @@
 			return NULL;
 		}
 
-		fss[drive] = Fs;
+		fss[(unsigned char)drive] = Fs;
 	}
 
 	return OpenRoot(Fs);