libsysutils: fix null pointer and memory leak issue

In SocketClient::quoteArg function
1. Fix potential null pointer accessing issue
2. Fix potential memory leak introduced by realloc fail

Change-Id: I1ca0f9089290d43452e9a71428244545f4ed866b
Signed-off-by: Hong-Mei Li <a21834@motorola.com>

libsysutils/src/SocketClient.cpp

@@ -112,6 +112,12 @@ char *SocketClient::quoteArg(const char *arg) {
     char *result = (char *)malloc(len * 2 + 3);
     char *current = result;
     const char *end = arg + len;
+    char *oldresult;
+
+    if(result == NULL) {
+        SLOGW("malloc error (%s)", strerror(errno));
+        return NULL;
+    }
 
     *(current++) = '"';
     while (arg < end) {
@@ -125,8 +131,9 @@ char *SocketClient::quoteArg(const char *arg) {
     }
     *(current++) = '"';
     *(current++) = '\0';
+    oldresult = result; // save pointer in case realloc fails
     result = (char *)realloc(result, current-result);
-    return result;
+    return result ? result : oldresult;
 }