Some more null pointer checks, fix #186

Signed-off-by: Sara Damiano <sdamiano@stroudcenter.org>

src/TinyGsmClientM95.h

@@ -445,6 +445,7 @@ class TinyGsmM95 : public TinyGsmModem<TinyGsmM95>,
   }
 
   size_t modemRead(size_t size, uint8_t mux) {
+    if (!sockets[mux]) return 0;
     // TODO(?):  Does this work????
     // AT+QIRD=<id>,<sc>,<sid>,<len>
     // id = GPRS context number = 0, set in GPRS connect
@@ -482,6 +483,7 @@ class TinyGsmM95 : public TinyGsmModem<TinyGsmM95>,
     }
   }
 
+  // Not possible to check the number of characters remaining in buffer
   size_t modemGetAvailable(uint8_t) {
     return 0;
   }
@@ -618,6 +620,7 @@ class TinyGsmM95 : public TinyGsmModem<TinyGsmM95>,
 
  public:
   Stream&       stream;
+
  protected:
   GsmClientM95* sockets[TINY_GSM_MUX_COUNT];
   const char*   gsmNL = GSM_NL;