Fix potential memory leak in String8::set

Not an actual leak at the moment, since no clients call set more than
once, but you never know how a class might be used in the future.

Change-Id: I601bad02eb16d522af3216ef903a190741444e4c

apps/Term/jni/termExec.cpp

@@ -64,6 +64,9 @@ public:
     }
 
     void set(const char16_t* o, size_t numChars) {
+        if (mString) {
+            free(mString);
+        }
         mString = (char*) malloc(numChars + 1);
         for (size_t i = 0; i < numChars; i++) {
             mString[i] = (char) o[i];