Revert "Revert "Cleanup some numeric code.""

core/src/fxcrt/fx_basic_gcc.cpp

Index: core/src/fxcrt/fx_basic_gcc.cpp
diff --git a/core/src/fxcrt/fx_basic_gcc.cpp b/core/src/fxcrt/fx_basic_gcc.cpp
index 6f17482156be7436f0e53e55243b2e1a63548202..7bf0725816589bc2640b163d232072454b819e9c 100644
--- a/core/src/fxcrt/fx_basic_gcc.cpp
+++ b/core/src/fxcrt/fx_basic_gcc.cpp
@@ -5,6 +5,7 @@
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #include <limits>
+#include <cctype>
 
 #include "../../include/fxcrt/fx_ext.h"
 #include "../../include/fxcrt/fx_string.h"
@@ -12,21 +13,20 @@
 template <class T, class STR_T>
 T FXSYS_StrToInt(STR_T str) {
   FX_BOOL neg = FALSE;
-  if (str == NULL) {
+  if (!str)
     return 0;
-  }
+
   if (*str == '-') {
     neg = TRUE;
     str++;
   }
   T num = 0;
   while (*str) {
-    if ((*str) < '0' || (*str) > '9') {
+    if (!std::isdigit(*str))
       break;
-    }
-    if (num > (std::numeric_limits<T>::max() - 9) / 10) {
+    if (num > (std::numeric_limits<T>::max() - 9) / 10)
       break;
-    }
+
     num = num * 10 + (*str) - '0';
     str++;
   }