pgmspace: zero out memory in strncpy_P (#2633)

strncpy should write 'size' characters, padding with zeroes if src is
shorter than 'size'.

cores/esp8266/pgmspace.cpp

@@ -147,6 +147,7 @@ void* memmem_P(const void* buf, size_t bufSize, PGM_VOID_P findP, size_t findPSi
 
 
 char* strncpy_P(char* dest, PGM_P src, size_t size) {
+    bool size_known = (size != SIZE_IRRELEVANT);
     const char* read = src;
     char* write = dest;
     char ch = '.';
@@ -156,6 +157,14 @@ char* strncpy_P(char* dest, PGM_P src, size_t size) {
         *write++ = ch;
         size--;
     }
+    if (size_known)
+    {
+        while (size > 0)
+        {
+            *write++ = 0;
+            size--;
+        }
+    }
 
     return dest;
 }