improve handling of non-numeric values in isNumber<T> (#168)

* improve handling of non-numeric values in isNumber<T>

previously, an exception was thrown and caught when isNumber<T> was
called on a non-numeric Slice.
now, this exception is avoided and the case is handled with a simple
initial value type check instead.

* make it more efficient

Author: jsteemann

include/velocypack/SliceBase.h

@@ -338,9 +338,12 @@ struct SliceBase {
                    v <= kMax;
           }
 
-          int64_t v = getInt();
-          return (v >= static_cast<int64_t>((std::numeric_limits<T>::min)()) &&
-                  v <= static_cast<int64_t>((std::numeric_limits<T>::max)()));
+          if (isInteger()) {
+            int64_t v = getInt();
+            return (v >= static_cast<int64_t>((std::numeric_limits<T>::min)()) &&
+                    v <= static_cast<int64_t>((std::numeric_limits<T>::max)()));
+          }
+          return false;
         } else {
           // unsigned integral type
           if (isDouble()) {
@@ -354,9 +357,12 @@ struct SliceBase {
             return 0.0 <= v && v <= kMax;
           }
 
-          // may throw if value is < 0
-          uint64_t v = getUInt();
-          return (v <= static_cast<uint64_t>((std::numeric_limits<T>::max)()));
+          if (isInteger()) {
+            // may throw if value is < 0
+            uint64_t v = getUInt();
+            return (v <= static_cast<uint64_t>((std::numeric_limits<T>::max)()));
+          }
+          return false;
         }
       } else {
         // floating point type