Changed data size check to be positive length, not > 1 to fix

ResizableDoubleArray constructor failure on input array of
length 1.

JIRA: MATH-1252
Thanks to John Bay

src/changes/changes.xml

@@ -54,6 +54,9 @@ If the output is not quite correct, check for invisible trailing spaces!
     </release>
 
     <release version="4.0" date="XXXX-XX-XX" description="">
+      <action dev="psteitz" type="fix" issue="MATH-1252 due-to="John Bay"> <!-- backported to 3.6 -->
+        ResizableDoubleArray constructor does not work with double array of size 1.
+      </action>
       <action dev="erans" type="fix" issue="MATH-1251"> <!-- backported to 3.6 -->
         Fixed initial value of "number of calls" counter in class "KohonenUpdateAction"
         (package "o.a.c.m.ml.neuralnet.sofm").
@@ -65,7 +68,7 @@ If the output is not quite correct, check for invisible trailing spaces!
       <action dev="erans" type="fix" issue="MATH-1248" due-to="Chris Popp"> <!-- backported to 3.6 -->
         Removed unnecessary allocations in "BigFraction" (package "o.a.c.m.fraction").
       </action>
-      <action dev="psteitz" type="fix" issue="MATH-1245">
+      <action dev="psteitz" type="fix" issue="MATH-1245"> <!-- backported to 3.6 -->
         Fixed error in computing discrete distribution of D statistics for small-sample
         2-sample Kolmogorov-Smirnov tests. Error was causing incorrect p-values returned 
         by exactP and monteCarloP methods (used by default for small, mid-size samples).

src/main/java/org/apache/commons/math4/util/ResizableDoubleArray.java

@@ -287,7 +287,7 @@ public class ResizableDoubleArray implements DoubleArray, Serializable {
         numElements = 0;
         startIndex = 0;
 
-        if (data != null && data.length > 1) {
+        if (data != null && data.length > 0) {
             addElements(data);
         }
     }

src/test/java/org/apache/commons/math4/util/ResizableDoubleArrayTest.java

@@ -124,6 +124,13 @@ public class ResizableDoubleArrayTest extends DoubleArrayAbstractTest {
         ResizableDoubleArray copyDa = new ResizableDoubleArray(testDa);
         Assert.assertEquals(copyDa, testDa);
         Assert.assertEquals(testDa, copyDa);
+        
+        // JIRA: MATH-1252
+        final double[] values = {1};
+        testDa = new ResizableDoubleArray(values);
+        Assert.assertArrayEquals(values, testDa.getElements(), 0);
+        Assert.assertEquals(1, testDa.getNumElements());
+        Assert.assertEquals(1, testDa.getElement(0), 0);
     }
 
     @Test