mesa: silence MinGW 'may be unused uninitialized' warning in get.c

The warning happens on line 2114 for the memcpy(data, p, size) call.
I'm not sure why that generates the warning but not the earlier use
of p in the code.

Reviewed-by: Neha Bhende <bhenden@vmware.com>

src/mesa/main/get.c

@@ -2051,7 +2051,7 @@ _mesa_GetUnsignedBytevEXT(GLenum pname, GLubyte *data)
    const struct value_desc *d;
    union value v;
    int shift;
-   void *p;
+   void *p = NULL;
    GLsizei size;
    const char *func = "glGetUnsignedBytevEXT";