cleaned up signed/unsigned int issues.

src/classes/PMusrCanvas.cpp

@@ -6462,7 +6462,7 @@ UInt_t PMusrCanvas::GetNeededAccuracy(PMsrParamStructure param)
     }
 
     // find last significant digit
-    for (UInt_t i=strlen(str)-1; i>=0; i--) {
+    for (Int_t i=strlen(str)-1; i>=0; i--) {
       if (str[i] != '0') {
         if ((i-decimalPoint) < precLimit)
           accuracy = i-decimalPoint;