FIX: MOLD/FORM on zero percent value

Fixes: metaeducation/rebol-issues#2394

src/core/f-math.c

@@ -260,7 +260,7 @@
 	/* handle sign */
 	if (sgn) *cp++ = '-';
 
-	if (trim == DEC_MOLD_PERCENT) e += 2;
+	if (trim == DEC_MOLD_PERCENT && (digits_obtained > 1 || sig[0] != '0')) e += 2;
 
 	if ((e > decimal_digits) || (e <= -6)) {
 		/* e-format */

src/tests/run-tests.r3

@@ -15,6 +15,7 @@ dt [ ;- delta time
 	wrap load %units/map-test.r3
 	wrap load %units/integer-test.r3
 	wrap load %units/decimal-test.r3
+	wrap load %units/percent-test.r3
 	wrap load %units/money-test.r3
 	wrap load %units/power-test.r3
 	wrap load %units/mezz-crypt-test.r3

src/tests/units/percent-test.r3

@@ -0,0 +1,41 @@
+Rebol [
+	Title:   "Rebol3 percent test script"
+	Author:  "Oldes, Peter W A Wood"
+	File: 	 %decimal-test.r3
+	Tabs:	 4
+	Needs:   [%../quick-test-module.r3]
+]
+
+
+~~~start-file~~~ "percent"
+
+===start-group=== "form/mold"
+	--test-- "form"
+		--assert    "0%" = form   0%
+		--assert    "1%" = form   1%
+		--assert   "10%" = form  10%
+		--assert  "0.1%" = form 0.1%
+		--assert  "100%" = form 100%
+
+		--assert   "-0%" = form   -0%
+		--assert   "-1%" = form   -1%
+		--assert  "-10%" = form  -10%
+		--assert "-0.1%" = form -0.1%
+		--assert "-100%" = form -100%
+
+	--test-- "mold"
+		--assert    "0%" = mold   0%
+		--assert    "1%" = mold   1%
+		--assert   "10%" = mold  10%
+		--assert  "0.1%" = mold 0.1%
+		--assert  "100%" = mold 100%
+
+		--assert   "-0%" = mold   -0%
+		--assert   "-1%" = mold   -1%
+		--assert  "-10%" = mold  -10%
+		--assert "-0.1%" = mold -0.1%
+		--assert "-100%" = mold -100%
+===end-group===
+
+
+~~~end-file~~~