fix: update implementation to preserve signed zeros

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---

Author: kgryte

lib/node_modules/@stdlib/blas/ext/base/dnannsumpw/lib/sumpw.js

@@ -147,14 +147,14 @@ function sumpw( N, x, strideX, offsetX, out, strideOut, offsetOut ) {
 	}
 	if ( (N-i) <= BLOCKSIZE ) {
 		// Sum a block with 8 accumulators (by loop unrolling, we lower the effective blocksize to 16)...
-		s0 = 0.0;
-		s1 = 0.0;
-		s2 = 0.0;
-		s3 = 0.0;
-		s4 = 0.0;
-		s5 = 0.0;
-		s6 = 0.0;
-		s7 = 0.0;
+		s0 = -0.0; // note: negative zero in order to ensure sign preservation if all elements are negative zero
+		s1 = -0.0;
+		s2 = -0.0;
+		s3 = -0.0;
+		s4 = -0.0;
+		s5 = -0.0;
+		s6 = -0.0;
+		s7 = -0.0;
 
 		M = (N-i) % 8;
 		for ( ; i < N-M; i += 8 ) {