Fix benign overflow in ToneGenerator.cpp

while (count--) causes an unsigned overflow when count = 0, refactor the
condition to avoid this.

Change-Id: I91a8c93e6be715d80070a4c94c1de712cb659265
diff --git a/media/libmedia/ToneGenerator.cpp b/media/libmedia/ToneGenerator.cpp
index 2cc4685..db44079 100644
--- a/media/libmedia/ToneGenerator.cpp
+++ b/media/libmedia/ToneGenerator.cpp
@@ -1581,7 +1581,8 @@
         }
         long dec = lAmplitude/count;
         // loop generation
-        while (count--) {
+        while (count) {
+            count--;
             Sample = ((lA1 * lS1) >> S_Q14) - lS2;
             // shift delay
             lS2 = lS1;
@@ -1592,7 +1593,8 @@
         }
     } else {
         // loop generation
-        while (count--) {
+        while (count) {
+            count--;
             Sample = ((lA1 * lS1) >> S_Q14) - lS2;
             // shift delay
             lS2 = lS1;